How can both internal and external email addresses have the same domain?
My company domain of email@example.com is different than my isp which is firstname.lastname@example.org. att isn’t going to allow my company use att.net and my company could not use mycompany.com if we were using the isp email address. Can you explain this a little more clearly? Perhaps this is why there is a problem.
Here the only solution i see at the moment is to rebuild the Exchange domains with somethin like yourdomain.local instead of the real yourdomain.com and then have the POP3 connector download the messages to the user’s mailboxes.
You can then set the default SMTP address to be yourdomain.com instead of yourdomain.local.
This is the final solution I found from Microsoft Support. I will test it as soon as possible but it’s exactly the problem I have: E-mail to external recipients with the same e-mail domain causes NDR messages when using the POP3 Connector