The MailJet add-on for Newsletter provides the integration with MailJet mail delivery service. MailJet provides a mail delivery service (via SMTP or API) you can use to reliably send emails from your Newsletter plugin, monitor them, have a bounce detection and reporting service (completely integrated with Newsletter). You can create an account on MailJet and try out the service for free (at time of writing the free account is limited to 200 emails per day).