Message routing question when aggregating

[Oman]

I run a server that hosts both a local mail domain (my.mail.com) and also acts as an aggregator for other mail services for the local users by using imapsync and/or fetchmail.

I have virtual domains setup for the other domains (other.mail.com) and the users have identities setup for when they reply to aggregated messages.

This all works extremely well except for one case:

if localuser1@my.mail.com tries to send a message to someotheruser@other.mail.com Zimbra tries to deliver it to a local mailbox since other.mail.com is setup as a virtual domain. The user someotheruser@other.mail.com does not exist locally.

Is there any way to force external routing of mail destined to a virtual domain?

If I don't setup virtual domains then I can't get the identities to work correctly. Should I be structuring the aggregating solution a different way?

Thanks
Jon