ISP port blocking practice

Paul Stewart pstewart at
Fri Sep 3 22:22:33 UTC 2010

It's extremely effective for us (not a large provider by any means).  We
block outbound 25 on all dynamic IP customers - to date it's never been
a problem for our customers.  Customer's who have static assignments are
not blocked by default.


-----Original Message-----
From: John R. Levine [mailto:johnl at] 
Sent: Friday, September 03, 2010 3:20 PM
To: Owen DeLong
Cc: nanog at
Subject: Re: ISP port blocking practice

>> 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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