ISP port blocking practice

Patrick W. Gilmore patrick at
Mon Sep 6 00:11:16 UTC 2010

Composed on a virtual keyboard, please forgive typos. 

On Sep 6, 2010, at 1:36, Claudio Lapidus <clapidus at> wrote:

> Hello all,
> On Fri, Sep 3, 2010 at 11:30 PM, Ricky Beam <jfbeam at> wrote:
>> 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...
> That won't be probably the case. Here recently we conducted a rather
> comprehensive analysis on dns activity from subscribers, and we've
> found that in IP ranges that already have outgoing 25 blocked we were
> still getting complaints about originating spam. It turned out that
> the bots also know how to send through webmail, so port 25 blocking
> renders ineffective there.

I believe you have confused "not 100% effective" with "ineffective".  And webmail is but one additional vector.  Bots know how to use smarthosts, corporate e-mail, triangulation, etc.  If you gave up on each because one step did not solve the problem, you would have no chance at a solution. 

When you unblocked port 25, did spam complaints go up or down?  There are a great many providers who have evidence that port 25 blocking lowers complaints even if there are bots that know their way around it. 

Second, assume you can wave a magic wand and block all webmail access.  Do you honestly believe the bots will not use port 25 to send spam directly?

Security requires layers.  And it is a bit shocking how many people do not realize this.


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