Function filter_on_server 

fn filter_on_server(
    s: &Session,
    config: &Config,
    account: &Account,
    new: &[PathBuf],
)

Your mail filters on account’s server after a fetch (new: its inbox’s arrivals, maybe none), as every device that fetches runs them (docs/client.md, “Filters”): what they did, and what they could not do, each in a line. Nothing sioul watch tells waits for them: it tells codes alone, which no filter touches.