Generally, you would need to add a filter on your router (access-list) so it does not allow TCP connections to the messenger service by blocking the TCP port listed and also port 80. Keep in mind that MSN may change the Ip address from time to time and this will require you to add/change the access-list so your internal users are unable to access the updated servers.
If you had a proxy server this will be the preferred way of blocking this type of service.
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...