Our mail server currently has a entry for NAT (197.187.4.240) binded to
the Pubilc interface and 197.187.5.50 (Exchange) to the private. There
are filter sets that pass traffic port 25 from public to Exchange.

We have a new spam filter applicance (197.187.5.72) that needs to sit
between Exchange and the internet. I have added a NAT entry for this
197.187.4.72 (Public) to 197.187.5.72 (PRIVATE).

Do I need to simply adjust my filters to specify the new appliance which
will be a mail relay or do I need to adjust the NAT table and bind
197.187.4.240 to 197.187.5.72 (new mail host)?