What should I do if I get a case like this ..

A is the head office, there are already domain and mail server but not using zimbra mail,
B is a branch office, and has not had a mail server, currently using a central server in office A,

if I install zimbra mail it should be the same whether his or her domain name must be different? because I want to keep the same domain as long as we use the same domain name, and whether the names of user B's in A should be in the clear and migrate to B. or is there another way

and if I want to create a mail server in the branch B using zimbra, what should I do to mail communications in local B to A should not be used, unless there is some sort mail from external domains from yahoo or google,.

requested by the very explanation ..