Please advise me in the following situation.

Our partner asked us about an year ago to set up a local domain with internal mailing, with domain like foo.local and mailing server mail.foo.local. The domain controller is a windows server, authentication on zimbra is done by active directory. The system was working very well and we are satisfied with zimbra.

This week they asked us to extend this setup so all the users can send emails to external addresses as well. We could add a second alias domain, but having two addresses can be a bit useless and confusing for the users in the internal mailing. Changing (renaming) the domain might pose problems i do not anticipate yet.

How should I go about this?