creating a second smtp server

[Suhaime Raeze <suhaime@ramunia.com>]

dear all appreciate for any opinions, howtos on the following problem

I have a primary MX which resides in my HQ office with a 1.5 Mbps line  
(HQ) where the users are about 150. I have also another site (Site A) 
where the user are about 500 users and have no physical connection to 
the HQ, Users in site A connects to the mail server via internet  
(2mbps) for POP connection and SMTP services.
Question:

How can i set up a second MX server that will reside at site A so that 
the people in site A will send to this local MX?  This MX server will 
then forwarded the  mails to the primary MX in HQ. I think this will 
eliminate the large number of concurrent connection to the HQ MX.
I am using Auth SMTP for user to send emails, so the question is do i 
have to create all the users mailboxes in the site A MX?
TQ