I recently did some work for somebody, and as a side, he asked me if it was possible to have postfix forward mail from one server to another for the same domain. For example, let's say his domain is example.com and he has an MX record that points to linux.example.com. He has some mailboxes hosted on linux.example.com and other ones hosted on windows.example.com. He wants any undeliverable mail that reaches linux.example.com to be forwarded to windows.example.com, still @example.com, of course.
- linux.example.com hosts email@example.com
- windows.example.com hosts firstname.lastname@example.org
- MX records point to linux.example.com
- A user sends an e-mail to email@example.com. Postfix (on the GNU/Linux machine) sees that it doesn't have a user named tom and registers an undeliverable. Rather than responding to the requestor with an undeliverable message, the user wants the data to be forwarded on to windows.example.com.
He is willing to create a record for every e-mail that resides on the Windows machine if necessary, but he would prefer that it always just forward undeliverables to the Windows machine so that he doesn't have to create a record every time he makes a new mailbox on the Windows server.