how to integrate with external non-zimbra server ?

We have two mail servers - one is for local mail, it lives in ourdomain.local, and another one is for external mail, it lives somewhere in the internet and serves anotherdomain.com
Currently we use exchange2003 for local mail but we want to move to zimbra
Idea is that when you send message to someone inside local domain, your mail is signed with user@ourdomain.local, so all mail stays inside, but as soon as you send a message to someone in the internet (for example to someone@gmail.com), this mail is signed with user@anotherdomain.com. Anotherdomain.com is linked with ourdomain.local via pop3 push-service, so all answers from internet are transmited from .com to .local
I have an idea how to organize .com to .local transmission, but i definitely have no idea how to change @domains depending on recepient position
In exchange it's rather simple - user has email alias for external email address and server has a build-in rule to use it when sending abroad local domain, but i can't find such rules in zimbra :-(
Any way to make it work ?
p.s. canonical address is not a good idea - if user@local have canonical user@com and sends mail to someone inside local domain, than all replies will be sent through external server :-( Adding a "from" aliace is also a bad idea because user have to change signs manually :-(