Mail¶

In short¶

The Mail page keeps all your mail, from every address, for when you choose to look; what is new waits on the Porch first. Sioul checks each message itself as it arrives, and tells you before you read it whether it really comes from the address it shows. Nothing in a message loads or runs, and an attachment goes through your system's antivirus, when it has one, before it opens. You write in plain text, in Markdown if you like, and whatever you send, move or delete can be undone for ten seconds. Sioul shows no unread counts, no badges and no red.

Protected by default¶

Without any setting:

• Every message is checked as it arrives. You see whether it is verified, not verified or forged before you read it. Forged mail, and mail borrowing a bank's or a public service's name, wait apart on the Porch with the reason (Reading).
• Nothing in a message loads or runs, so nobody learns that you opened it, and every link shows where it goes before it opens (Reading).
• Attachments go through your computer's antivirus before they open, and a program attached is never started. Without an antivirus Sioul asks first; on a phone, which has none Sioul can call, it says the file is not checked (Attachments).
• Your mail travels encrypted. Sioul has no setting to talk to a mail server in clear.
• Nothing leaves without you, and you have ten seconds to change your mind. No read receipt is ever sent. There is no Sioul server: your mail goes between your device and your provider, and nowhere else unless you choose to send it somewhere (Privacy and security).

Once you set them up:

• Your own spam filter, trained on your computer from your mail; your mail goes to no filtering service (Your own spam filter).
• Signed and encrypted mail with OpenPGP, your key in your system's keyring or on a security key (Signing and encrypting).

Folders¶

• On the left, each address with its main folders: Inbox, Sent, Drafts, Archive, Junk, Trash. The others are folded under More folders.
• A small dot marks a folder with something new. How many is said in words when you open it.
• A folder shows its last two weeks. Earlier messages opens the rest, so that no list is endless.
• Search, at the top of a folder: by sender, recipient and subject. More…, beside it, searches everywhere, by conditions (below).
• By conversation, in the Mail page's ⚙: a message and its answers together, under the newest one; your own answers come from Sent.

A right click on a folder offers Keep a copy here or Stop keeping a copy (the server keeps every message either way), New folder…, and Delete this empty folder, for an empty folder of yours.

Reading¶

A message opens on the right; the folders fold away while you read.

First, who sent it, and how far that is proven. Sioul checks every message itself as it arrives, by asking the sender's domain whether the message is really its own:

• verified: the domain vouches for it;
• not verified: nothing proves it either way, and the reason is said;
• forged: the domain says it did not send it, and asks that such mail be kept apart. Forged mail is set aside on the Porch, with the reason, and never deleted.

A message that borrows the name of a bank, a public service or your own domain, even written with lookalike letters, is set aside as well. Hovering over the shield beside the sender shows each check (how a sender is checked).

Then its text, shown safely. HTML mail keeps its paragraphs, lists, bold text and links, and nothing else: no pictures, no styles, no scripts. Nothing loads from the network and nothing runs, so no tracking picture learns that you opened the message. Earlier messages that a reply quotes are folded under Show the earlier messages; a signature is dimmed.

Every link says where it goes before it opens: its site in bold and its full address, under the message, while the pointer is on it. On a touch screen, the first tap shows the address and the second opens it. Only web and mail links open; a mail link opens a new message in Sioul, to that address alone.

Above the message, always in the same place:

• Reply, Reply to all (only when there are others to answer), Forward;
• Add ▾ (a task, an event, a note, a reply, the sender to your contacts) and Link to… (anything else: a task, a note, a project);
• Archive, Delete, Junk;
• Unsubscribe, on a newsletter or a list's message that says how to leave it (below).

Archiving, deleting and junking happen at once, with Undo in the status line for ten seconds; the server is told only after that. There is no "are you sure?". In the trash, Delete for good deletes it for good, with the same ten seconds.

What you mark Junk, or Not junk in the junk folder, teaches your own spam filter at its next training (below).

A right click on a message (or its ⋮, or the Menu key) gives the rest: mark as read or unread, flag, Move to…, Show the source, block the sender, How they reach you… (their list, Always through, what reaches you from them: A person's sheet), Keep as a contract….

Opening a message marks it read, as any mail program does. Sioul never sends a read receipt.

Several at once¶

• Ctrl + click adds a message to the selection or takes it out; Shift + click takes everything since the last one clicked; Ctrl+A takes all; Esc none. On a touch screen, a long press opens a message's menu: Select, then a tap chooses more, or leaves one.
• A bar then marks them read, archives, deletes or moves them, under one Undo, however many there are.
• Drag them onto any folder of any address, with a mouse or a touchpad: the folder under the pointer is outlined. From a search's results too, which is how a busy inbox is sorted: the folders come back in the search's place while you drag. Into another address, a message is taken off the first one only once the second server has it.
• Move to…, in the bar or in a message's menu (right click, ⋮, or the Menu key), does the same from the keyboard, and on a phone.

Unsubscribing¶

A newsletter or a list's message that says how to leave it has Unsubscribe above it. One click, and ten seconds later Sioul does what the list asks:

• it tells the list's server, when the list offers this and its signature proves the request is the list's own: the "one click" most newsletters offer;
• else it sends the list the message it asks for, from the address the newsletter came to; the message goes to Sent like any other;
• else, when only a web page can do it, it opens that page in your browser, at once: you finish there.

Undo stays in the status line for those ten seconds. Then the line says "Unsubscribed from Type & Pixels.", or why not. The message itself stays where it is.

The button rests, dimmed, on mail that is forged, set aside as spam or called probably spam by your own filter, in the junk folder, borrowing a name, hostile, from a sender you blocked, or when nothing proves that the address it names belongs to the list: answering such mail would tell its sender that your address is read, or reach someone else. Its tip, or a tap on a phone, says why.

A list you left shows Unsubscribed, and when. If its mail keeps coming, block the sender: ⋮ ▸ How they reach you… ▸ Blocked. The lists you left are in the Mail page's ⚙, under Lists you left, on this device.

Invitations¶

A message that brings an invitation shows it above the text: Accept, Maybe, Decline. The event goes into your calendar and your answer goes to the organiser. A published event has Add to my calendar; a cancelled one, Take it out of my calendar.

Attachments¶

Attachments are folded under the message, one line each with their kind, name and size.

• Your system's antivirus checks each one before it opens or is saved: Microsoft Defender on Windows, ClamAV on Linux and macOS when it is installed. When it finds a threat, nothing opens and the copy is deleted.
• A program, a script, a shortcut or an installer is never started from a mail, checked or not: save it if you trust it, and run it yourself.
• Without an antivirus, Sioul does not refuse: it says the file will not be checked, asks before opening it (Open it unchecked), and says how to get one.
• A file larger than the antivirus scans is never called clean: Sioul says it was not scanned because it is too big, and asks the same way.
• On a phone, which has no antivirus Sioul can call, attachments are not checked, and Sioul says so in the message and after each opening, rather than asking each time. An attachment opens in the app you choose (Android asks which, when none is set for its kind of file), which can read that one file and nothing else of Sioul's; Android's installers (.apk) never open, as programs never do. Save… asks Android where to put the file.
• On Windows and macOS, files you open or save carry the system's mark that they came from the Internet, so that the system treats them with its usual care: Office opens them in Protected View, macOS checks them before they run.
• The attachments of mail set aside do not open.
• Keep in papers files one in your papers: checked, then kept, and you say what it is.

Writing¶

Write is always in the same place. You write in Markdown, and line breaks are kept as you type them, as in a chat: a blank line starts a paragraph. Preview shows the message as it will look.

• Cc, Bcc are folded until you need them. When you have several addresses, From chooses which one sends; an answer goes from the address it answers.
• Attach, or drop files on the window. A paper, beside it, attaches one of your papers; one that has ended, or is older than usually asked, says so in the list.
• Your signature is put below the text when the draft starts, after the usual "-- " line, so that you see what goes out. Your name and signature are set per address in Accounts (Name and signature…).
• Answering, the window says "Below your text: Camille's message of …, quoted". The quote is added when the message leaves.
• Drafts are saved as you type. Closing the window loses nothing: the draft waits under Drafts, on the left. Drafts stay on your devices: they reach your other devices sealed when you share between them, and never go to your server's Drafts folder.

Send (or Ctrl+Enter) waits ten seconds, with Undo, before the message leaves. Then a copy goes to Sent. Nothing is ever sent without you.

The message leaves as HTML for those who read HTML, and as plain text, the Markdown itself, for the others.

From other apps¶

• On a phone, Sioul is in Android's share sheet: share a photo, a PDF, a link or some text to Sioul, and a new message opens with it, from your usual address. Your addresses are there too, each on its own (Android 10 and later): choose one, and the message goes from it. A long press on Sioul's icon shows them as well, Write from ….
• Files are copied into Sioul as they come. The window says "Attaching 2 files…" until they are there, and Send waits for them. The copies go once the message is sent or deleted.
• Mail links (mailto:), in a browser or another app, open a new message in Sioul, with its address, subject and text, when Sioul is your mail app. On a phone, Android asks which app opens them the first time. On Linux, choose Sioul as your mail program: KDE, System Settings ▸ Default Applications; GNOME, Settings ▸ Apps ▸ Default Apps. A link clicked while Sioul is open opens there. Not yet on Windows and macOS.
• Nothing is sent until you press Send. Other apps see nothing of your accounts but the addresses in the share sheet.

Signing and encrypting¶

Encryption is not there for its own sake: it is what lets the letters that offices now keep behind their portals come back to your mail, readable by you alone (why).

When you have an OpenPGP key (made or imported in Accounts ▸ Encryption), the writing window has two small switches, Sign and Encrypt.

• Sign lets the people you write to check that the message is yours and was not changed on the way.
• Encrypt makes the message readable only by them and by you. It works when Sioul knows every recipient's key; otherwise it says whose key is missing, and Look for their keys asks for them. Sioul looks only when you press it, since looking tells a server to whom you write.
• The subject is not encrypted: anyone who carries the message can read it. Keep it plain.
• Your messages carry your public key, so that people who write back can encrypt.
• Answering an encrypted message keeps it encrypted: Sioul switches Encrypt on when it can, and opens the message you answer only when you press Send, with the same PIN as the signature. Unencrypted, your answer goes without the quote, and a forward does not go. What you type in your answer is kept as any draft is: in clear on each of your devices, and sealed between them.

An encrypted message is decrypted when you open it; a signature is checked and said under the sender: "Encrypted · Signed by …", or "The signature does not match the text", in warm colours, never red.

On a computer, your key can live on a security key, such as a YubiKey, which never gives it out (below).

In quiet time¶

Outside working hours, work addresses fold, without their dots: "Work mail rests until work comes back. It is all here if you look for it." While you sleep, every address folds: "While you sleep, mail rests: nothing notifies, and the Porch shows only what your lists let through now. The rest is all here if you look for it." See Hours.

Not there yet¶

Emptying the trash or the junk in one go, copying a message to a folder, saving it as a file, drafts kept on the server, sending later, an encrypted subject, revoking or extending an OpenPGP key, security keys on a phone, lookalike domain names. They are planned. Sioul has no S/MIME.

Going further¶

Searching¶

The field at the top of a folder finds a sender, a recipient or a subject in that folder. When you need more, More…, beside it, opens the search by conditions in the folders' place. Nothing of it shows until then.

• One condition at first, Anywhere, with what you had typed in the folder's search. Add a condition for another: the sender, the recipients, the subject, the text, an attachment (there is one, none, one with a name), the kind of attachment (a PDF, a picture…), the day it arrived, its size, who the sender is to you, a newsletter or a list, the address, the folder, and whether it is read, flagged or answered. Case and accents do not matter.
• With two conditions or more, choose Mail that meets all of them or any of them.
• The results take the list's place, the newest first, each saying where it is. A message opens beside them as in a folder, so you go back and forth between the results and the messages. Above them, a sentence says what is searched: "Mail from …@bank.example…, arrived after 3 June, with an attachment."
• Everywhere: every folder of every address, but the junk and the trash, unless you name them in a Folder condition.
• On the servers too: Sioul keeps your recent mail here, and brings older mail as the disk has room; folders you keep on the server only are not here at all. A moment after you stop typing (at once with Enter), Sioul asks your servers for the rest, and says so: "Looking on the servers…", then "3 more on the servers.", or why a server could not be searched. Those messages say "on the server"; opening one brings it here first. Asking a server marks nothing read. A server minds accents more than Sioul does: write them as the messages do.
• Clear goes back to the folder.
• Make it a filter…, beside it, turns the search into a mail filter: its editor opens in the Mail page's ⚙, to choose what it does with such mail as it arrives.

The conditions are the ones mail filters use, in the same words.

Filters¶

Filters act on new mail as it arrives, on its server: into a folder, archived, to spam, flagged, marked read, to the trash, or with a keyword. They are in the Mail page's ⚙, under Filters: one list for all your addresses, each filter in a sentence.

• Add a filter opens it in place, with one condition and one action. Choose what the condition reads (From, To, Cc, Reply-To, Subject, Text, Anywhere, an attachment, its kind, who the sender is to you, a newsletter or a list, the day or the time it arrived, its size), how it compares (contains, does not contain, is, is not; before, after, between; larger, smaller), and what it looks for. Add a condition for another; with two or more, choose whether every condition holds or one of them holds. Case and accents do not matter, as in the search.
• Then: move it to a folder, archive it, mark it as spam, flag it, mark it as read, move it to the trash, or add a keyword. Add an action for another. Filters do not forward or answer mail.
• A sentence says the filter as you build it, and what is missing if anything is: "Choose the folder it moves messages into." Every change is kept at once.
• Folded under one line: On which addresses it acts (every address, or some), and Once it acts, the filters below are not asked.
• Try it on the inboxes counts what it takes in your inboxes now, read or not, and names a few.

• Each filter in the list has its switch (off, it is kept and does nothing), its sentence, ▴ and ▾ to ask it earlier or later, and the pencil to change it. The order matters: the first filter that moves a message decides where it goes. With several addresses, Show the filters of shows those of one address.
• When they act: on new mail that arrives unread in an inbox, once, on whichever of your devices fetches it first (this computer, your phone in the background, sioul watch); your other devices then leave it alone. If that device stops half-way (closed, out of battery), another one finishes the work ten minutes later. Your filters travel to your other devices with your settings.
• What they never touch: mail the Porch sets aside, a code you asked for, what your own spam filter caught. Nothing is deleted for good: the trash keeps it.
• Not told: a message a filter moves out of the inbox, or marks read, is not notified, and does not wait on the Porch. When a filter cannot act on a message (a folder missing, the server refusing), the message is notified as any new mail, and the status line says why.
• Run them on the inboxes… applies them to everything in your inboxes now, read mail too. Sioul first says what would change; Run them now does it after ten seconds, with Undo.

• From a search: Make it a filter…, beside the search's Clear, makes a filter of its conditions and opens it at the end of the list, for you to choose what it does.

Your key on a security key¶

On a computer, your OpenPGP key can stay on a security key (a YubiKey, a Nitrokey), which signs and decrypts by itself and never gives the key out. Set it up in Accounts ▸ Security key.

A message your security key signs is signed when you press Send, before the ten seconds of Undo: a band at the bottom of the writing window asks for the key's PIN, and says how many tries are left when some were lost; then, when the key asks for a touch, it says to touch it now and hold the finger on it a second or two, as the key waits about fifteen seconds and a brief tap is often not taken. Not plugged in, it says so, and goes on as soon as the key comes. Send unsigned and Not now are there all along, and Undo throws the signed message away and opens the draft again. The PIN stays in memory fifteen minutes after its last use, never written or logged anywhere; it is forgotten when you pull the key out, close Sioul, or choose Forget the PIN now in Accounts.

A message encrypted to your security key is never opened just because it is shown: the line under the sender says "Encrypted for your security key", with Open with your security key. Once opened, it opens again without the key, its attachments too, until Sioul closes.

Not yet on a phone.

Settings¶

The ⚙ at the top of the Mail page:

• By conversation: messages grouped with their answers.
• Fetch every: how often the folders other than the inbox are fetched. The inbox comes as soon as the server says something arrived.
• Font, Size, Line spacing: how messages read. The same three are in the Porch's ⚙, and behind Aa in Notes.
• Lists you left: each list you left from a message, when and how, once there is one.
• Filters: what is done to new mail on its server, by conditions (above).
• Your own spam filter: below.

Each address's own settings (what it is for, how far back, how often, its protection) are on its card in Accounts.

Real time, at the top of the page, fetches every folder of every address each minute, for a code or a password you are waiting for.

Your own spam filter¶

Under Your own spam filter, in the Mail page's ⚙. It judges strangers' mail only, never that of someone you know, a code you asked for, a project's mail or your own. What it does on the Porch: Spam, and your own filter.

• What it does with each verdict: for what it finds probably spam, maybe spam and probably not spam, one of three:

  • Move to spam: as it arrives, the message goes into its address's Junk folder, on the server, where your other mail apps see it too; it waits on the Porch, in Caught by your own spam filter, where Not spam brings it back to the inbox. Only mail as it arrives: what is in the inbox already stays there, flagged;
  • Flag only: it stays where it is, marked, and waits there too;
  • Do nothing: it goes to its lane, as any message.

  Until you choose: probable spam and doubts flagged, nothing done with the rest. Nothing it flags or moves is ever notified, on any device. - Spam from: how sure it must be to call a stranger's message probably spam, 95% unless you change it. Higher: fewer of your messages taken for spam, more spam let through. - Maybe spam from: from here up to the other, a stranger's message is maybe spam; below, probably not; 50% unless you change it. Each slider stops short of the other: "maybe" stays below "spam". - Its training, on one computer: the first time by hand, with Train now, then again by itself once a week (below). Train it on that one alone (two computers training their own is not supported). Train now names the addresses it learns from, and pressing it is your consent to it, for each (Microsoft's terms ask it for a Microsoft address): Sioul keeps the list of the addresses named at your last press, and training again by itself, or for an AI agent, learns from a Microsoft address only when it is on that list. It never learns from a work or school Microsoft address, which the sentence names apart. It downloads what training needs from every folder of every address but the trash, drafts and sent mail (the headers, the names of attachments and the start of each text, never the attachments themselves), without changing anything on the server, then learns from your mail: your junk folders teach it most; what it caught itself counts as it is (what it moved or found probably spam, as spam; a maybe spam, only once you say); and what you said is spam or not, on any of your devices, always wins. The first time takes long. While it runs, it says where it is ("Messages fetched: 1,200 of about 15,000", then each step), and Sioul stays usable; Stop keeps what came, and the next time goes on from there. Its new table replaces the one in use only if it takes no more of your messages for spam. Below, the last training: when, whether its table replaced the one in use and why, from how many messages, and on your newest mail how much of it was taken for spam and how much spam was caught, each with its margin; then what it learns from, the room left on the disk, and the outside material you imported, if any, with what the filter measured on it. - Train again by itself once a week, when this computer is plugged in and idle: on unless you switch it off, on the computer that made the table in use; on your other computers the switch is greyed, and one sentence says which computer trains. Once a week at most after the last training, by hand or by itself; never on battery nor while saving power, only after 15 minutes without anyone at the computer or with its session locked, never during a focus session. It runs at the lowest priority, on half the processor; on a metered connection it learns from what it has, without downloading first. Unplugged or saving power while it runs, it stops and tries again later. It never notifies you: one line under the switch says when it last trained by itself, and whether its table replaced the one in use. - Outside material: mail labelled elsewhere (an older filter's archive), imported once on that computer with sioul spam import <file>, gives the filter more words to learn and a baseline to measure it against; it stays on that computer, apart from your mail, and sioul spam import --remove <name> takes it away. Its format: the developers' notes, "Outside material". - Its table, on a phone, which never trains it: the computer that trained it, and when, with what it measured then. It comes through your folder, sealed (the part Spam filter, in Sharing), with what you said is spam or not on each device, and what its filter caught there.

What it does with each verdict, the thresholds and whether it trains by itself travel to your other devices with your settings. What training reads, and what it keeps: Privacy and security.

Compared with other apps¶

As of October 2026, from each app's own documentation.

✓ documented · partly, with a note · ✗ not found in the app's own documentation (for Sioul: not built) · ? not confirmed · — not applicable.

Thunderbird is the free-software mail program for computers, with an Android app; Gmail, Outlook and Apple Mail are the most used; Proton Mail is the reference for encrypted mail; FairEmail, a free-software Android app that shows each message's checks, joins the technical table.

For technical readers¶

Which addresses work¶

Any IMAP and SMTP server that takes a password over an encrypted connection: TLS from the first byte (ports 993 and 465, as RFC 8314 prefers) or STARTTLS (143 and 587), never a plain connection; a server that does not offer STARTTLS is refused. Certificates are checked against your system's (rustls, TLS 1.2 and 1.3). IMAP4rev2 (RFC 9051) and IMAP4rev1, with IDLE (RFC 2177, renewed every five minutes), MOVE, UIDPLUS and SPECIAL-USE; each address is kept as a Maildir, whose files only you can read on Linux and macOS.

Gmail and Google Workspace take an app password, or Sign in with Google with a Google key of your own (OAuth 2.0 for native apps, PKCE, SASL XOAUTH2), not yet tried against Google itself (Gmail and Google Workspace). Outlook.com, Hotmail, Live, MSN and Microsoft 365 addresses sign in on Microsoft's page the same way, with no secret at all (Sioul is a public client), not yet tried against Microsoft itself (Outlook.com, Hotmail and Microsoft 365). JMAP, POP3 and Exchange's own protocols are not supported. More in Works with.

How a sender is checked¶

When a message is fetched, Sioul checks it itself, through your system's DNS (Stalwart's mail-auth), within twenty seconds:

• SPF (RFC 7208), on the server that handed the message to your provider: the first public address from the top of the Received lines, since every line below it could be written by anyone;
• DKIM (RFC 6376), each signature against the key its domain publishes, at arrival, since keys change and some signatures expire within days;
• DMARC (RFC 7489), with the domain's own policy;
• ARC (RFC 8617), for forwarded mail, and the sending server's reverse DNS.

The results are written on top of the stored message as an Authentication-Results header (RFC 8601), under a name only your copy of Sioul uses, ending in .invalid, so that no sender can write results that pass for Sioul's. Sioul trusts them first, then your provider's own header, whose name it learns from your mail. A message too large to fetch whole is checked on its headers: DKIM and ARC are then left unsaid, and DMARC is said only when it passes.

• Verified: DMARC passes for the domain shown in From, or a valid DKIM signature of that same domain. A signature of another domain proves nothing about the sender.
• Forged: DMARC fails, and the domain asks receivers to act on it (p=quarantine or p=reject). With p=none, or through a mailing list, which rewrites what it relays, the sender is only not verified. No list relays your codes: a code that fails DMARC under a policy is forged, list headers or not.
• Not authenticated: SPF and DKIM both failed, and neither DMARC nor an ARC chain sealed by your provider or one of your domains vouches for it. Nothing proves it comes from the address it shows, so its sender counts as a stranger for every rule of the Porch: screened, judged by spam filters, no project lane by address.

Hovering over the shield shows each check, its result, the domain it was for, and who made it: Sioul at arrival, or your provider. Your provider's spam verdict (SpamAssassin's or rspamd's headers) is read only from the lines it wrote itself, above the one where the message came in.

Borrowed names¶

A signature proves a domain, not the name written before the address. Sioul compares the name shown with about fifty brands and public services, most of them French (the tax office, health insurance, family benefits, fines, employment, the post and parcel services, energy suppliers, telephone operators, banks) beside large platforms (Amazon, PayPal, Netflix, Apple, Google, Microsoft), and with your own domains, after reducing both to a skeleton of lookalike letters (Unicode's confusable detection, UTS #39: l for I, 0 for O, rn for m, Cyrillic letters). A name claiming one of them from an address outside its domains is set aside; a sender you know, or let in, keeps the name they use. Large shared providers, such as gmail.com, are nobody's own domain. Lookalike domain names themselves are not checked yet.

Mail shown safely¶

HTML is cleaned with an allowlist (ammonia): text structure and links only; styles, scripts and the page's head removed with their content, comments too; no attribute but a link's address; only http, https and mailto links; relative links refused, since they would name a file or a share of your computer. Pictures are never loaded, and there is no setting to load them. A link's site is read after any name@ put before it to mislead (https://bank.example@other.example shows other.example), and a mailto: link opens a draft to its address alone, without the subject or text it would fill in.

The antivirus¶

On Windows, the Antimalware Scan Interface (AMSI) hands the file to the antivirus Windows runs, Microsoft Defender by default; this path follows Microsoft's documented sequence and is built by the project's automatic builds, but has not yet run on a Windows computer. On Linux and macOS, ClamAV when the system has it: its daemon (clamdscan --fdpass), else its scanner with the system's signatures, else with signatures Sioul keeps current once a day with the system's freshclam. Nothing of ClamAV ships with Sioul. A Flatpak cannot reach the system's ClamAV, and asks. Programs (.exe, .js, .lnk, .desktop, .iso…) are only ever saved. An attachment's name is cleaned before it is written: no folder in it, and on Windows no device name or hidden stream. Files opened or saved carry Windows' Mark of the Web (Zone.Identifier, zone 3) or macOS's quarantine flag (com.apple.quarantine). On Android nothing is scanned: Android offers no way to have another app scan a file, and ClamAV does not run there. The attachment goes to the app you choose by a content:// address lent to read only, for that one file; its type comes from its name, never from the message, and is never Android's installer.

OpenPGP¶

Sioul uses Sequoia (OpenPGP, RFC 9580) with its pure-Rust cryptography and standard policy, the same on every system. Messages go out as PGP/MIME (RFC 3156): signed alone as multipart/signed, otherwise multipart/encrypted with the signature inside, encrypted to each recipient and to you, so that you can read it again in Sent. Inline PGP is read too. A decrypted message lives only in memory. A key made in Sioul is Curve25519 (EdDSA to sign, ECDH to encrypt), valid three years, its passphrase made at random and kept in your keyring. Its revocation certificate, which declares the key no longer to be used if it is ever lost or stolen, is kept beside it, in pgp/own/ of Sioul's data folder (~/.local/share/sioul/ on Linux), readable by you alone; Save the revocation certificate, beside the key in Accounts, puts a copy in your downloads, to keep somewhere safe, apart from your device. Anyone who has it can revoke your key. Sioul cannot revoke a key yet: the certificate waits for that day. Others' keys come from their messages (Autocrypt, never from forged mail, so that a forged message cannot slip in a key for someone you write to), from a file, or, when you ask, from their domain's Web Key Directory, then keys.openpgp.org, over HTTPS. Your own keys stay on the device they were made or imported on; others' public keys travel to your other devices, sealed. Not yet: protected headers (the subject stays readable), revoking or extending a key, S/MIME.

Security keys¶

An OpenPGP card (specification 3.4: a YubiKey, a Nitrokey), reached through the system's smart card service (PC/SC: pcscd on Linux, the Smart Card service on Windows, CryptoTokenKit on macOS). The card signs a digest or opens a session key itself; its private keys never leave it, and each signature it makes is checked against your certificate before use. The PIN is typed in Sioul, held encrypted in memory, forgotten fifteen minutes after its last use, when the key is pulled out or Sioul closes, and never written or logged: Sioul installs no logger, and caps logging as it starts, since the card library's traces would include the PIN. The card is reset after each operation, so that no other program finds it unlocked. Sioul never asks for the Admin PIN and never makes or loads keys on the card. Tested against a software card, and with a real YubiKey 5 on Linux on 8 October 2026: set up, renewed through GnuPG, signing and encrypting at Send, opening encrypted mail; not yet with a real key on Windows or macOS. On a phone, the same card, held against it (NFC, Android's IsoDep) or plugged into its USB-C port (CCID over Android's USB host API, its smart card interface alone): the PIN first, typed for each use and never kept, then one visit that reads the key, checks it, gives the PIN, signs or opens, and locks it again (VERIFY reset, OpenPGP card 3.4.1 §7.2.2); tested against a stand-in card, not yet with a real key. A key with one PIN try left is given the PIN only once you typed it again, told. The limit: Sioul gives the PIN only to a key that shows the identity and fingerprints of yours, but these are public, so a counterfeit card made to copy them, within reach of the phone or plugged in, would receive the PIN (never a private key).

The spam filter¶

A fastText classifier, trained on a computer on your own mail: the words, and facts from the headers, among them Sioul's own SPF, DKIM, DMARC, ARC and reverse-DNS results. It is calibrated (Platt), then folded into a small table of hashed words, with no word in plain text, which every device reads, your phone too, and which travels sealed. A new table replaces the one in use only when it takes no more good mail for spam on your newest mail. Its training reads your mail without changing anything on the server, and keeps what it read on that computer. More: Privacy and security and the developers' notes.

Unsubscribing in one click¶

One click (RFC 8058) is used only when a DKIM signature that passed covers both List-Unsubscribe and List-Unsubscribe-Post, and belongs to the domain in From or to the address's own; it posts the form alone over HTTPS, to a name on the Internet, without cookies or anything of yours, follows redirects only within the same site, and gives up after fifteen seconds. Otherwise Sioul sends the message the list asks for (RFC 2369), from your address, or opens the list's page. Two List-Unsubscribe headers, or an address it cannot use, and it does nothing.

Search and filters on the server¶

A search asks the servers with UID SEARCH for the folders and days this device does not hold, checks what they find on its headers here, and never marks anything read (BODY.PEEK). On Gmail, attachments are found with Gmail's own search. Before a filter acts, the device marks the message with the keyword $SioulFiltered, with CONDSTORE (RFC 7162) when the server offers it, so that two of your devices never act on the same message. Servers without keywords (Exchange, Outlook.com) cannot carry the mark; there, what a filter does twice leaves the message as once.

What Sioul writes, and what it sends¶

Fetching changes nothing on the server (EXAMINE, BODY.PEEK). Sioul writes there only when you act, or when your filters and your spam filter do what you set them to do: a read mark when you open a message, a move ten seconds after you asked, a copy in Sent, a keyword ($Junk, $NotJunk, $SioulFiltered). When it sends, it introduces itself as [127.0.0.1], so that your computer's name does not enter the message's Received lines; it writes no X-Mailer, and makes the Message-ID under your own address's domain. It never sends a read receipt.