Function drain_mail_outbox 

pub async fn drain_mail_outbox<E, Dispatch, DispatchFut>(
    config: &MailOutboxDispatchConfig,
    dispatch: Dispatch,
) -> Result<DispatchStats, E>
where
    E: Display,
    Dispatch: FnMut() -> DispatchFut,
    DispatchFut: Future<Output = Result<DispatchStats, E>>,

Runs mail outbox dispatch passes until no rows are claimed or the configured drain limit is hit.