ISP port blocking practice

From: "Dobbins, Roland" <>
Date: Fri, 3 Sep 2010 21:07:49 +0000

> Could you point to more than one instance? I've not yet found one.

I've yet to run across this, either, FWIW, except on extremely
restrictive special-purpose endpoint networks. Doesn't mean that it
doesn't happen, but it doesn't seem to be nearly as prevalent as
TCP/25 blockage on general SP access networks.

Worst case I have seen was the visitors network at EBC at one of the
nation's largest telephone and Internet transit providers. They seemed
to block ALL outgoing ports except 80, 443, and 22. No VPN. No
submission port. No IMAP. (Didn't try POP3.) I tunneled mail over ssh,
but I can imagine that a lot of corporate types who meet there are
rather annoyed that they can't access mail.