Function after_fetch 

pub(crate) fn after_fetch(
    account: &Account,
    new: &[PathBuf],
    first: bool,
) -> Filtered

After a fetch of account (its inbox’s arrivals, new; none when only something waits for another look): the filters on the server, on a thread that may wait for it. What a filter could not act on is to be told as any new mail, the notifications having held it back for them (untold).