A little mail server help !

[Anmol Bedi]

Hi,

I have been asked to setup a mail server (POSTFIX).

We have 4 locational offices in same town.

And I have to setup 5 servers (4 locations + 1 headoffice)

And my user criteria is : 

1. First group : This group's users can only mail to the locational office people and no one else.

2. Second group : This group's users can mail in location office as well as other locations and head office

3. Third group : This group's users can mail anyone.


Can anybody help me out what type of access files would I be making at every locations for implementation of postfix.

Regards,
Anmol Bedi