I configured postfix on my CentOS 5 machine to masquerade my entire private LAN Microsoft Exchange domain to my public internet domain so that sent Exchange email can be routed on the internet.

# vi /etc/postfix/main.cf
…
smtp_generic_maps = hash:/etc/postfix/generic
…

vi /etc/postfix/generic
…
@msexchangedomain @publicdomain
…

postmap hash:/etc/postfix/generic
service postfix restart