I have a Mail server (FQDN: mail.ours.com) using Postfix and Dovecot, users having email address like user@ours.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 amy@ours.com want to send email to bob@external.com, but Bob do the setting to let his email forward to bob@ours.com. So SMTP server from external.com will interact with ours SMTP server and claim that the sender is from amy@ours.com wanted to send mail to bob@ours.com. After the request, SMTP from external.com will receive the error message from ours SMTP server said that “(amy@ours.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.

  • 2
    I have never encountered a mail server that was configured in that way. It looks like you are trying to (poorly) reinvent SPF/DKIM... – fkraiem Mar 19 '19 at 4:24
  • Thanks for the comment, can you kindly tell me more detail about your reply. I'm not trying to block any client from our mail server, i think it's reasonable to reject someone who claim that he is valid user in our mail domain but cannot prove his identity. (Because mail server from external.com cannot prove that she is amy@ours.com, so our mail server reject the request with "sender not login", but if she is amy@others.com, everything will be fine), thanks again for the reply! – Michael Ouyang Mar 19 '19 at 8:25
  • That's exactly what DKIM is for; look into it. – fkraiem Mar 19 '19 at 8:28
  • Thanks again for the comment, but DKIM seem not work for the situation I encounter. Below is the process 1. amy@ours.com want to send email to bob@external.com 2. BOB(bob@external.com) "forward" the mail to bob@ours.com 3. external SMTP server make the request to ours SMTP server (MAIL FROM: amy@ours.com, RCPT TO: bob@ours.com) 4. ours SMTP server reject the request according to the setting(reject_sender_login_mismatch in smtpd_sender_restrictions) with Error Message(<amy@ours.com> sender not login) How can I accept this request make by external SMTP server. Very Thanks! – Michael Ouyang Apr 14 '19 at 12:46
  • The point is that since the mail would be properly DKIM-signed, your server should accept it regardless of where it comes from. smtp_server_restrictions is not the proper mechanism for what you are trying to do. – fkraiem Apr 14 '19 at 13:27

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