Function mail_shown 

pub(super) fn mail_shown(
    s: &Session,
    shield: Option<&Shield>,
    key: &Path,
    subject: &str,
    detail: &str,
) -> (String, String)

A message’s title and detail (its sender) as an agent sees them, wherever it is listed: its subject masked with what its text says (a code in the subject, its phrase below); nothing of either when it is hostile mail to a shielded address.