consumers: https://wiki.debian.org/Debate/DefaultMTA
- ssmtp - no queue
- http://askubuntu.com/a/735071 see man for revaliases, or, better user msmtp
- msmtp - no queue
- nullmailer - has queue ?
- no local mail, 329192 -> Lacks mechanisms for responding to permanent failures, backing off retry frequency and timing out messages in the queue.
- https://opensourcehacker.com/2013/01/02/sendmail-using-nullmailer-and-gmail-account-on-linux-server/ (xinetd, 12.04)
other: https://wiki.debian.org/GmailAndExim4 issue: If you send a mail to a mailing list via a SMTP server provided by Gmail, the returned mail message from the mailing list is treated as read by the Gmail POP3 service
http://websistent.com/how-to-use-msmtp-with-gmail-yahoo-and-php-mail/ (smarthost vs satellite)
Elastic Email - impressed me with their API: https://api.elasticemail.com/public/help