ISP port blocking practice

Brett Frankenberger rbf+nanog at
Tue Sep 7 00:55:06 UTC 2010

On Mon, Sep 06, 2010 at 10:38:15PM +0000, deleskie at wrote:
> Having worked in past @ 3 large ISPs with residential customer pools
> I can tell you we saw a very direct drop in spam issues when we
> blocked port 25.

No one is disputing that.  Or, at least, I'm not disputing that.  I'm
questioning whether or not the *Internet* has experienced any decrease
in aggregate spam as a result of ISPs blocking port 25.  Did the spam
you blocked disappear, or did it all get sent some other way?  I'm
questioning the evidence for the claim that it didn't just all get sent
some other way.  (By "all", I mean "almost all".  I'm sure at least one
piece of spam has been permanently prevented from getting sent as a
result of port 25 filters.)

I'm not suggesting ISPs should or shouldn't block port 25.  That's a
decision each must make for itself.  And I'm not questioning that
blocking offers benefits for those who choose to block it.  What I'm
questioning is whether or not it results in any meaningful reduction in
aggregate spam.  (That is, are people *receiving* less spam becuase the
three ISPs you describe above -- along with many others -- blocked port

     -- Brett

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