ISP port blocking practice

John R. Levine johnl at
Fri Sep 3 22:19:47 UTC 2010

>> It's been extremely effective in blocking spam sent by spambots on
>> large ISPs.  It's not a magic anti-spam bullet.  (If you know one,
>> please let us know.)
> That simply hasn't been my experience. I still get lots of spam from booted hosts in large provider networks, and yes, that includes many that block 25. As near as I can tell, 25 blocking is not affecting spammers at all, just legitimate users.

I know people at large ISPs with actual data.  Port 25 blocking is quite 


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