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 firstname.lastname@example.org while the second postfix also accepts mail for email@example.com.
I added a forwarding rule to postfix saying that postfix should forward mail destined for firstname.lastname@example.org to email@example.com (for regular local delivery) and to firstname.lastname@example.org. 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@example.com, 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?