Blocking port 25 outbound for dynamic users until they specifically request it be unblocked seems to me to meet the "no undue burden" test; so would port 22 and 23. Beyond that, I'd probably be hesitant until I either started getting a significant number of abuse reports about a certain flavor of traffic that I had reason to believe was used by only a tiny minority of my own users.
Sorry, I must've missed something. Port 25 outbound (excepting ISP SMTP server) seems entirely logical to me. Port 22 outbound? And 23? Telnet and SSH _outbound_ cause that much of a concern? I can only assume it's to stop clients exploited boxen being used to anonymise further telnet/ssh attempts - but have to admit this discussion is the first i've heard of it being done 'en masse'. It'd frustrate me if I jacked into a friends Internet in order to do some legitimate SSH based server administration, I imagine... Is this not 'reaching' or is there a genuine benefit in blocking these ports as well? Mark.