After using transfer tool to transfer a single cPanel account from one server to another, transfer tool failed to detect the email routing settings for some of the domains, and the weirdest thing happened: no routing setting was set for the domain names.
As a consequence, all emails sent to these domains is rejected with the following error message:
Message not delivered550 Please turn on SMTP Authentication in your mail client. mail-yw1-f48.google.com [22.214.171.124]:32858 is not permitted to relay through this server without authentication.”
To fix this, simply head over to cPanel > Email Routing and recheck the “Auto Detect” or “Local”
Select the domain name and re-set the desired option:
After doing that the emails should start receiving normally.