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 
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- has a non-specific relationship to
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MDL-48000 Incoming email improvements for ease of setup
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- Open
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