Mail routing on the Intranet / Internet

[Fabio da Silva Cunha <fsc_(_at_)_mymail_(_dot_)_com_(_dot_)_br>]

Hi Friends!

I have a litle problem: 

        I have a small intranet (3 Sites conected by leased lines with
routers) and i need to connect it 
to the internet to send and receive e-mail. 

        All Sites have one OpenBSD 2.4 Mail Server with Sendmail 8.8.8, and
uses private address.

        The Head Office uses a OpenBSD 2.4 (Sendmail 8.8.8) with two network
interfaces: one private 
conected to the intranet, and one public, conected to the internet by the
firewall (OpenBSD 2.4).
        

        My network:
                                           |---------------|
           loc1.domain.com.br              |               |
        ---------------------- 10.1.1.x    | Head Office   |
        | mail server 1      |-------------|               |
        ----------------------             |               |
                                           |               |
           loc2.domain.com.br              | domain.com.br |
        ---------------------- 10.1.2.x    |               |  200.x.x.x
[--------]
        | mail server 2      |-------------|   Mail
|--------------[FIREWALL]----> INTERNET
        ----------------------             |   Server4     |
[--------]
                                           |               |
           loc3.domain.com.br              |               |
        ---------------------- 10.1.3.x    |               |
        | mail server 3      |-------------|               |
        ----------------------             |---------------|

        The problem:

        How to route mail beetwen these 3 locations and the internet through
the Mail Server4 in Head Office.

        Any ideas?

        Thanks in advance!

        Regards,

        Fábio da Silva Cunha