I have a Mail server (FQDN: mail.ours.com) using Postfix and Dovecot, users having email address like email@example.com. Like many others SMTP server, if user claim that they are from @ours.com, they need to pass sasl or should be in mynetwork parameter according to the restriction setting; so if user do not pass sasl and not having the ip address in mynetwork, they may got some error message like “sender not login”.
The situation is when a user firstname.lastname@example.org want to send email to email@example.com, but Bob do the setting to let his email forward to firstname.lastname@example.org. So SMTP server from external.com will interact with ours SMTP server and claim that the sender is from email@example.com wanted to send mail to firstname.lastname@example.org. After the request, SMTP from external.com will receive the error message from ours SMTP server said that “(email@example.com) sender not login”, and Amy will finally got this bounce error message.
I have no idea how to deal with this situation, very appreciate if anyone can give me the solution or leave some comment, thanks.