It would be extremely useful to let the administrator test the Incoming mail configuration before putting it into production.
https://moodle.org/mod/forum/discuss.php?d=277594#p1193531 is a good example where a Moodle administrator should test the plus addressing support before going in production with this feature enabled.
A simple "Test button"|"Test me checkbox" returning the plain SMTP response to a RCPT TO: <email_test_plus_addressingsupport> command, in fixed chars, - or limiting the plain response disclosure only when the server response differs from 250 - under Site administration > Server > Incoming mail configuration > Mail settings would save tons of headaches .
- Sends an email to itself
- Get's picked up on next cron task run - are this step can be manually run from the gui too
- Does basic check that the email arrived and record some sort of latency metric between sending and receiving end to end
- Have an additional optional admin setting for asserting the state of the email is in the correct shape. Probably something very simple like a text area and each line is a regex which is matched against the headers. This can then be configured to test that the incoming email server has done the spf / dkim / dmarc checks. OR alternatively we recheck all these headers ourselves in php land (if the bundled phpmailer library can do this?). We'd then want a setting to say 'check passes if [ spf / dkim / dmarc / all ] pass. My preference is to get the email server to do this rather than php as there is a few different levels of strictness around these specs