Our company just acquired company A, and the email address is @a.com runing on MS Exchange. Our company's email address is @x.com, and is running on Domino Lotus Notes.
The management wants company A to use our email address(eg. firstname.lastname@example.org) but still keep their SMTP server for communiting with existing cuetomer, so we set mail forwarding in Lotus Notes. For example, mail sent to email@example.com, will be forwarded to firstname.lastname@example.org. We also asked users in company to set their "reply to" email address to @x.com, so it's totally transparent to the recipient.
The problem is when the employee of company A reply the email received from their peers (Company A), then it will reply to @x.com. The mail goes to company A's SMTP, then route to company X's SMTP. Finally, company X will forward the mail back to Company A, because of mail forwarding. By doing so,this is putting extra load to both SMTP servers. Both companies in different network, any better idea to make MS Exchange recognized that the email from local domain eventhough the "reply to" email address in @x.com?