[Dshield] ISP's not blocking egress 25/tcp

Matthew Sullivan matthew at sorbs.net
Thu Jan 22 12:42:46 GMT 2004

Erik van Straten wrote:

>Hi Chuck, SORBS, list,
>On Wed, 21 Jan 2004 15:24:24 -0500 Chuck Lewis wrote:
>>Also, and you smarter folks here please correct me, I think I've read/heard
>>that a real easy way to vastly reduce spam is if ISP's would validate the
>>address(s) that stuff comes from ?
>It's not that simple. Currently backdoored DSL PC's directly submit
>spam to the intended recipient's mailserver, entirely bypassing the
>PC's Inet Service Provider MTA (Mail Transfer Agent, mailserver).
>ISP's could force the use of their MTA's by blocking outbound SMTP
>traffic (some already do), but some customers (like me) may want to
>run their own MTA. Many customers object to any port-block, so it is
>not in the interest of ISP's to do this (unless they get sued more
>often for not taking action on misbehaving customers).
>Also, blocking 25/tcp egress traffic and forcing the use of an
>address like <joe@*.isp.tld> prevents customers from legitimately
>using other sitenames, and may break mail forwarding (which,
>unfortunately, spf.pobox.com does too). However, issues like these
>can probably be fixed (I'm not sure about SPF).
>Actually the situation is worse. If ISP's block egress 25/tcp and
>force the use of their MTA's, then spammers will be pushed to use
>that route if they backdoor a PC behind such a block. I can assure
>you, they will (the ISP could introduce egress rate-limiting and
>filtering which would be an emormous improvement). However, spammers
>do send spam to spamtraps, which gets the ISP's mailserver(s) listed
>on blacklists. One such "misstep" tends to suffice.
>Some email blacklists are totally unreasonable, and therefore
>counterproductive. They do not distinguish between backdoored DSL
>PC's and mailservers that send huge amounts of legitimate mail on a
>daily basis (like listservers). Therefore ISP's will be hesitant to
>even consider blocking egress SMTP traffic. Thus, those type of
>blacklists effectively do NOT help fighting spam; IMHO They Make
>Things Worse.
>| http://www.dnsbl.au.sorbs.net/cgi-bin/lookup?IP=
>| Click: Check Entry
>| Scroll down to "Database of servers sending to spamtrap addresses"
>| Click one of the "Detail" buttons
>| Click Yes on the security alert (untrusted CA and hostname mismatch)
I really will have to fix that at some point... :-/

>| Read what it says
>Note: = mailhost1.tudelft.nl, one of the main Delft
>University mailservers.
>We do not live in a perfect world. We do have compromised PC's on our
>campus network, and we're fighting like h*ll to get these fixed, and
>educate users.
>Note that many spamtraps are horribly misconfigured. They respond to
>spams that come with spoofed originator addresses, and do NOT supply
>a null return-path. Because of the spoofed originator address, the
>bounce (generated by the spamtrap) is sent to an MTA that was NOT
>involved before. If that bounce cannot be delivered, that MTA *must*
>(RFC2821) inform the sender, which is the spamtrap. Kaboom.
For the record SORBS does not list bounce mail as spam, nor virus laden 
emails (some do get through) when alerted to a listing caused by a 
bounce or a virus/trojan hitting a spamtrap it is immediately removed. 
(This does not include spam sent via trojans, only the trojans 
themselves)  We have protection in to stop such listings but is 
enivitable that some will get through.

>So it is possible that the spam that caused to get
>blacklisted, originated from a PC not on our campus, was sent to
>us by the spamtrap, and was legitimately returned to that spamtrap
>because it could not be delivered on our campus. But then, it may
>also very well have been a compromised PC or server on our campus,
>that did send spam via
Received: from mailhost1.tudelft.nl ([]) by mta185.mail.scd.yahoo.com
with SMTP; Fri, 28 Nov 2003 14:40:01 -0800
Received: from aoj.com (x076059.lr-s.tudelft.nl []) by
mailhost1.tudelft.nl (Postfix) with ESMTP id E9D8D65C5; Fri, 28 Nov 2003 23:39:59
+0100 (CET)

Looks like one of the tudelft.nl PCs to me.....

>I have been advocating blocking egress 25/tcp traffic on our campus,
>with the exception of some legitimate mailservers (like mine :).
>However, because of some issues (that can probably be fixed), and
>perhaps because our main MTA is blacklisted, this measure is not yet
>effective (it certainly does not help). Thank you SORBS.
>Is anyone aware of advantages or complications of blocking outbound
>SMTP that I missed?
>Note: it is not my intention to start a flamewar which blacklist is
>good/bad. Anyone who wants to do that, at least change the subject.
I do actually beliving in what you are saying and please read: 
http://www.merit.edu/mail.archives/nanog/2003-12/msg00300.html whilst 
this system is not complete it will be coming online soon.



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