Function smtp_builder 

fn smtp_builder<'a>(
    account: &Account,
    host: &'a str,
    credentials: Option<(&'a str, &'a str)>,
) -> Result<SmtpClientBuilder<&'a str>, SyncError>

The sending server’s client, logged in with credentials when given (an account signed in with OAuth logs in after, sasl::smtp_xoauth2).