It sounds like you need to use the altsrchost command to set up Virtual Gateway mappings. You can specify that messages that match a source IP or a domain will be sent out using a specific interface. I believe altsrchost has to be done from the CLI and not the GUI. The basic setup is pretty easy, map a specific sender IP or envelope sender domain to a specific interface to be used on exit.
Check out the section "Using Virtual Gateways" in Chapter 2 of the Advanced Configuration Guide as there are a lot of options that can be set up.
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...