If you have entered a one-to-one static NAT entry for smtp port, exchange will go outside from the public IP that the static is applied. If you forwarded port 25 to inside mail server by using PAT, you may not be able to achieve what you want. Posting related sanitized config will help us to determine more clearly.
The simple solution is not to use a 'different' public-ip when sending outbound email. You are right many internet hosts will do a reverse lookup of your hostname before letting you send email (e.g hotmail/msn).
Since you have a default route to another interface than the the one that has desired public IP, and current Cisco firewall devices do not support Policy Based Routing, what you want to achieve is not possible. But here are some workarounds.
Do not use different public IP for mail server as Farrukh suggested and request for MX record change from your hosting provider.
Use a SmartHost service from an ISP or Hosting provider, configure your exchange to send and receive over that SmartHost, then add a route statement into firewall to route traffic destined to smarthost IP to the interface that has your desired public IP.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...