A Domino server (Server A) receives Internet mail for abc.com and must pass that mail to Server B (Server B, belongs to another division using MS Exchange, and a separate Internet email domain) internally via SMTP instead using a connector application. Server B is behind the firewall and out of contact with the Internet. Server A receives SMTP mail for email@example.com and looks for him in the Domino directory. It decides that that "abc.com" mail should go to Server B. The problem is that all of your Domino servers in the Domino domain use Server A to route SMTP addressed mail to all Internet domains. Therefore, Server A also has this route configured and loops mail on itself before returning a delivery failure to the sender with no way to route to abc.com.
We can work around this:
1) On Server A, set the Mail System (in the Person doc) to be "Other Internet Mail" and the Forwarding address field as jdoe@ServerB.yourcompany.com.
2) Set the first entry in the Username field of the Person document (Server A) to be firstname.lastname@example.org. This ensures that Notes type-ahead and Notes name resolution will use SMTP syntax when looking up a name for internal memo creation.
3) Set up a Foreign SMTP domain for ServerB.yourcompany.com
4) Setup a connection document using the Foreign SMTP Domain name in the Destination Domain field with the SMTP MTA Relay Host field set to ServerB.yourcompany.com.
Now, Internet mail for email@example.com comes to Server A, sees his forwarding address as ServerB.yourcompany.com, and sends the mail over to Server B since Server B is known as a Relay Host, though it is not really relaying in this case. On Server B, "jdoe" is found and mail is delivered.
When jdoe sends mail out, his address is assigned on Server B (his mail server), not Server A, and keeps this address as it passes thru Server A on it's way to the Internet. His recipient sees the proper Internet email address, firstname.lastname@example.org, ensuring that Internet recipients and your company's email users (if it's internal mail delivery) always see the sender as email@example.com.