ISP port blocking practice

Owen DeLong owen at
Sat Sep 4 00:20:25 UTC 2010

Sent from my iPad

On Sep 4, 2010, at 7:49 AM, "John R. Levine" <johnl at> wrote:

>>> 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 effective.
Does the data show that blocking was effective, as in the host didn't detect the block and proceed around it, or, merely that lots of hosts try the direct approach first?

Merely tripping over the filter, even repeatedly does not in and of itself prove efficacy.


> R's,
> John

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