In order to migrate to a new mailserver with little dns problems/downtime, I have set up a second postfix that is currently accessible on a subdomain mx record, eg. the main postfix accepts mail for [email protected] while the second postfix also accepts mail for [email protected].

I added a forwarding rule to postfix saying that postfix should forward mail destined for [email protected] to [email protected] (for regular local delivery) and to [email protected]. Local delivery still works as expected, but when trying forward the mail to the new mx, postfix appeds the domain part at the end of the forwarding address, resulting in [email protected], which of course fails and the mail bounces.

Why does postfix mess with the alias name in that way and how can I turn that of?