Let the administrator or cron check the inbound email plus addressing support

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 .

Check api:

1. Sends an email to itself
2. Get's picked up on next cron task run - are this step can be manually run from the gui too
3. Does basic check that the email arrived and record some sort of latency metric between sending and receiving end to end
4. 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