Hi Michael,
Thanks for your suggestion, but it seems to give the same error it gives when I use ./discourse-doctor.
root@discourse-app:/var/www/discourse# rake 'emails:test[brian.lui@foo.co]'
Testing sending to brian.lui@foo.co using foo.mail.protection.outlook.com:25, username:user@foo.co with login auth.
======================================== ERROR ========================================
UNEXPECTED ERROR
Net::SMTPSyntaxError
====================================== SOLUTION =======================================
This is not a common error. No recommended solution exists!
Please report the exact error message above to https://meta.discourse.org/
(And a solution, if you find one!)
=======================================================================================
Brian