I have sender checks disabled for a mailbox user, so they can send mail from a sender address not hosted by my mailcow server. It is an external address that gets incoming mail forwarded to my mailcow mailbox.
Now I want mailcow to send all mail that is sent by the mailbox user, using the external address in the From address, through a certain transport.
However, with the sender-dependent transport feature, it seems I can only manage to send all mail sent by a mailbox user through a selected transport, regardless of the used From address.
How can I achieve a sender-dependent transport based on an eternal alias of a mailbox user?