Comment 14 for bug 891811

This worked for my YM account. Empathy had it configured to use port 5050. I changed it to 23.

It could have been a work firewall issue. Apparently, valid choices include 20, 23, 25, 80, 119, 5050, 8001, and 8002. Incidentally, I saw exactly the same behavior in Pidgin.

http://sherylcanter.com/wordpress/2008/11/configuring-pidgin-behind-firewall/