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 .