Thnx for your answer, but I just found the solution
in the admin-interface -> configuration -> server -> MTA
there you can set a different SmartRelay then the global one. There is even a button "reset to global"
I'm sure I checked every setting in the admin-post 10 times before doing my post here in the forum, so I guess this settings just turned up after I posted
No: After I resetted this value to the global value zimbra is relaying through my new smtp-server.
peter