I have many IP addresses on my "Outside" NIC for multiple Websites, FTP Servers, etc. All of my domains incoming SMTP point to the same IP address and work just fine for inbound mail. My outbound mail is all NAT'd by ISA to a single IP address that I can not control. Who cares right? Well, with more and more SPAM fighters doing Reverse DNS lookups, I do. I can make my RDNS PTR match my domains mx a record, but now that means all of my clients web browsing AND all of my SMTP traffic is on the same IP.
I would like to be able to static NAT my inside mail server to a specific outside IP address.
The first one is already explained by the original poster. If your emails are getting whacked by remote email servers performing reverse DNS lookups, you can bind the IP number of your emails server (the one on the MX record) as the primary IP number on ISA's external NIC. Now your email server's IP number will be properly resolved (RDNS) by the remote SMTP server. The disadvantage (or inconvenience), as the original poster explained, is that any outbound traffic will use your email server's IP number as source. For various reasons, you may want to avoid this setup.
The second approach, which Tom briefly mentioned, involves new External or DMZ Network. You supposed to move your mail relay to this Network and assign the public IP of the MX record to your email server. You should then create an access rule allowing your mail relay to send emails to External Network. You should take the second approach with the grain of salt. I haven't done this myself. I am just repeating what others have posted in this forum. One of these days when I have time, I will test this out in my lab network.