Our client is using exchange 2003, installed on a DC with ABC.com as there FQDN. They are popping their ISP for selected mail with the ISP assigning them mail.ABC.com as their SMTP and POP servers. Mail sent from the internet is arriving fine. If a user wants to forward it to another user internally using Outlook 2000 client, it never arrives and there is no NDR. In fact I have never seen an NDR. I suspect the emails are arriving in the bad mail folder.
Internal default address is user@ABC.com
ISP address is user2@ABC.com
recipient policy is @ABC.com
How exactly does Exchange 2003 use SMTP to route internal email to outlook clients?