Function smtp_sign_in_with_tokens 

async fn smtp_sign_in_with_tokens<T, C, CF, K, KF>(
    login: &str,
    connect: C,
    token: K,
    oauth: OAuth,
) -> Result<SmtpClient<T>, SyncError>
where
    T: AsyncRead + AsyncWrite + Unpin,
    C: Fn() -> CF,
    CF: Future<Output = Result<SmtpClient<T>, SyncError>>,
    K: Fn(bool) -> KF,
    KF: Future<Output = Result<String, SyncError>>,

AUTH XOAUTH2 on connections connect makes, with the tokens token gives: the one kept first; refused, a fresh one, once; refused again, the access must be given again. A refusal no token changes (Microsoft’s Authenticated SMTP switched off) is said at once (OAuth::smtp_refused).