FYI as dedicated zimbra mail server taken the port 80
so u unable to use others domain inside the server but there ia a way to do that, by pointing zimbra to use another port eg, 81
then you have to use apache virtualhost and proxy pass to point MAIL domain to use port 81 instead of port 80 , and reserve port 80 to ur sub/domain
i can provide step by step if this is the thing u need because i have done this on my mail server , user login will not need to care about the port
cheers
Routers