I’ve setup a mailcow with two accounts (a@a.com and b@b.com) for two domains (a.com && b.com) where other accounts of the b.com domain are managed on another server outside of mailcow.
For both domains I have a custom transport map in place so everything sent by a@a.com should use the official relay server of a.com and b@b.com should use the relay server of b.com. This works when sending from a@a.com or b@b.com to addresses outside of a.com and b.com because the correct relay server is used.
When sending an e-mail from a@a.com to anotheraccount@b.com the used relay server is b.com and not a.com which leads to a 550 of course.
How can I figure out why the wrong transport map is used?