Topic: How To Configure Internal Mail Server

I need configure one internal mailserver.
the scene is :

Have one domain (mydomain.com) configured in one ISP (Theplanet)
i have 20 emails accounts, all emails working fine .

I send around 12000 emails / day (internal) and 1000 emails / day(external) but  the internal msg size is 300Kb or more.

The solution need cath emails of my ISP through pop3 and redirect to my local account.
when i send internal email, route in the server
When i send external email, redirect the emal to the my ISP smtp server using smtp auth

have a possible ?

Patrick Peres