We use ZCS as internal communication channel for a group of users, in which we restricted their outside send/receive mails operation, so that they can only communicate with the users in the Zimbra domain.

However, another group of users they have their own office email address (different from the Zimbra domain), also need to communicate with this group of users. In the past, we just create another zimbra a/c for this office users, and then let them communicate with the "internal" users. Up to this moment, we have to change this approach, because the no. of office staff are increasing, and actually this approach is annoying to them (they have to keep two separate email a/c and both cannot communicate with each other).

My facing question is, can I create or put the office email address (say, @office.com) to the Zimbra Global addressbook (say, @internal.com), so that the users @internal.com using webmail can look up the email address of the users @office.com? Indeed, I tried but Zimbra do not allow me to create email address without a prior setup domain on server...

Any good suggestion and idea? Thanks for all your kindly help!!