[exim] Sending from MX to different server based on recipien…

I'm using exim4 as the MX server. The imap intranet server is being migrated from dovecot to zimbra.

I'd like to be able to route some addresses in the same domain to the zimbra server and some to the dovecot server, like this:

user1@??? --> MX --> dovecot
user2@??? ---/ \--> zimbra

Then I can migrate the users by groups (train them, and then go to the next). Right now all messages go to MX and then to dovecot.

I don't even know how to start, somebody pointed me to virtusertable but it looks pretty cryptic. Is there another way, or an example on virtusertable?

Thanks, Eduardo.