Help me guide to setup mail server as a mail-gateway use spamassasin (use sendmail)

Here is plan

I user centos 5 as sendmail gateway (user sendmail)

User MS exchange server for local network

_________ ____ ____
/ \
| internet | <------------>| |<----> |
\_________/ |_____________| |_______|
Sendmail server Internal Mail Server
Centos5 Exchange 2000
mail.domain.com int-mail.domain.com
@domain.com @abc.domain.com
192.168.100.20 192.168.100.30

When mail from Internet sendto @domain.com -> fw to @abc.domain.com.
All mail from: int (@int-mail.domain.com) to internet then renamed to @domain.com

im sorry i dont understand exactly what you are tryin to do can you give me more info and i will try and help (i dont know if you didnt give enough info or if its just me being dumb lol)