I am setting up a new Zimbra 8 FOSS server. It needs to receive mail for 2 domains names. Domain A and Domain B. In our previous mail server setup, both domain's MX records pointed to smtp.provider.com. In the previous setup, all the user@b.com accounts were aliases of user@a.com. In the new setup, I don't want them to be aliases. In looking through the documentation and wikis, it isn't clear to me how to do this. Some articles reference having to setup SSL certificates for the other domain. There is mention of needing Zimbra proxy. Can someone give me some clear direction? There will be some cases where I want the mailbox for user10@b.com to be shared with user2@a.com so that they can send and receive emails from both in the same pane of glass.

Thanks in advance,
Stephen