Function verify 

pub fn verify(
    accounts: &[Account],
    password: &dyn Fn(&Account) -> Result<String, SyncError>,
    dirs: &Dirs,
    progress: &mut dyn FnMut(&Progress),
    cancel: &Cancel,
) -> Update

Sioul’s own checks of the messages kept without them (Checked), and nothing else: nothing new is downloaded (sioul spam fetch --verify). Each message is fetched once by its UID, whole (or its header block, above CHECK_WHOLE), in memory only; its checks are appended beside the records after each group, so a run stopped goes on there. A message its server no longer has (its folder gone or renumbered, its UID gone) is gone: unknown for good. Progress: Verify, account after account.