understanding Jabber IM connections
"Assuming that the family elders have not disabled server-to-server communications between capulet.com and montague.net, Juliet's message is routed to the Jabber server at montague.net"
Symantec whitepaper on how IM routes around firewall rules
"Imagine that an instant messaging client is a fugitive on the run from the authorities. The fugitive wants to cross a police roadblock (a firewall) on the main highway to reach his safe house (the instant messaging server) and hide. Because the fugitive knows that the police are blocking traffic on all lanes of the highway, he decides to take the bike path next to the highway (HTTP, port 80). Since the police assume that only legitimate cyclists (Web surfers) will use the bike path, the fugitive can safely slip by the roadblocks and reach the safe house."