Sendonly - #670

I was managing mailservers for more than a decade long, and i would like you to consider some of my cases, and thoughts:

1. We had besides sendonly, also receiveonly mailboxes (for suspended customers, also a receiveonly + disabled login)
2. a sendonly like noreply should not REJECT, in case someone is protecting itself with a quick test for return delivery, i would rather opt for DISCARD.
3. BUT, if you discard for sendonly, every mailer-daemon kind of problem has to detected during initial sending, because any later feedback from mailer-daemon would be discarded too
4. To avoid the REJECT/DISCARD problems, we wrote a rather spartan noreply-handler, which got all the mails from noreply@ account piped, and parsed the email, so in case it was a recognized mailer-daemon we could store it, or better, we could set a temp or permanent undeliverable status on recipients of the original email.

You see, the sample case of "sendonly like noreply@" is mostly solved by other ways, so i would suggest to find cases that better fit as a sendonly mailbox.

One example i could think is to enable different sender accounts for the same mailaddress, so one can also control which user is alowed to send (by permitting the sender account to use that email as sender address)

Anyway i like some of your solutions, for example the sql query with REJECT / OK based on sender only