What version of 6.3 is this? If it's 6.3(1) then there is a known bug with MSN Messenger not working through the PIX (CSCeb45344, not visible from outside as it was found in pre-production testing). The fix is available in 6.3(2), but this code is not available due to another sever bug. You can open a TAC case and get the latest interim release of 6.3(2) made available to you for testing if you like.
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...
[toc:faq]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 and UDP are I...