ISP port blocking practice

Ricky Beam jfbeam at
Sat Sep 4 02:30:19 UTC 2010

On Fri, 03 Sep 2010 08:12:01 -0400, Owen DeLong <owen at> wrote:
> Really?  So, since so many ISPs are blocking port 25, there's lots less  
> spam hitting our networks?

Less than there could be.  It appears a lot less effective because there  
are so many ISPs not doing any blocking.  Both of my residential  
connections are open, and always have been. (even dialup was unblocked.   
which I always found odd since the UUNET wholesale dialup agreement  
requires the RADIUS response contain a packet filter limiting port 25 to  
your mail server(s).)

If I block port 25 on my network, no spam will originate from it.  
(probablly) The spammers will move on to a network that doesn't block  
their crap.  As long as there are such open networks, spam will be  
rampant.  If, overnight, every network filtered port 25, spam would all  
but disappear.  But spam would not completely disappear -- it would just  
be coming from known mailservers :-)  thus enters outbound scanning and  
the frustrated user complaints from poorly tuned systems...


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