Function act 

pub fn act(
    account: &Account,
    password: &str,
    file: &Path,
    action: &Action,
) -> Result<(), SyncError>

Does action to the message stored at file, on the server, then here. What it says of the message, junk or not, goes into this device’s label log (spam::labels) once the server has taken it; what your own filter moved, into this device’s log of its moves.