As far as I know, there is no way to do this in AsyncOS. You need to have your DNS servers resolve this address. If the internal mail server has an RFC1918 address then you'll need to configure your ESA to use your local DNS servers rather then the root servers. That's the only way to get correct resolution on locally administered addresses.
OK, thanks for pointing to the DNS setup. You can actually setup an alternate DNS server for a specific domain there, so I just needed to override my current (provider's) DNS servers for my reverse DNS zone (xx.xx.in-addr.arpa).
In my case, with only 2 internal mail servers relaying to the ESA, host entries would have been a simpler and more robust way to do it, but the DNS override does the job.
You can actually setup an alternate DNS server for a specific domain there
That's news to me, but I haven't looked at those settings for several years.
My outgoing Received headers now look pretty.
But if your IP addresses are indeed RFC1918, then anyone looking up the host name in those pretty headers will either get an address they can't use, or the name simply won't resolve. Either way, the correctness of the situation is debatable. At least "unknown" is correct from an external point of view.
If you're using publicly routable addresses then the question becomes, why won't your ISP either put in correct PTR records for you or delegate to you?
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...