En la siguiente página, se pueden ver los puertos que usa yahoo messenger.
Estos son:

Chat & Messenger      TCP Port 5050: Client Access only
Insider/Room Lists    TCP Port 80: Client Access only
File Transfer         TCP Port 80: Server Access.
                      Your ISP may block this port, as its used for web hosting.
                      You can change port in Messenger, Preferences, File Transfer.
Voice Chat            UDP 5000-5010
                      TCP 5000-5001: Client Access
                      If UDP Fails, TCP will be used instead, see below.
WebCam                TCP Port 5100: Client Access
Super Webcam          TCP Port 5100: Server Access
P2P Instant Messages  TCP Port 5101: Server Access
                      PMs between Buddys may not use the Yahoo! Server, 
                      but this is not a requirement.