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20 Sep 2006 gets blacklisted Email

I run a service called that provides a contact database for people to use to report spam and other network abuse. One of the ways people can use it is to register and then forward mail through it, so that for example mail to is remailed to whatever the abuse contact is for

Last Friday (while I was on the way to a meeting at an undisclosed location east of Seattle) someone sent me a note telling me that mail sent through was bouncing:

 Remote host said:
  554 Service unavailable; Client host []  blocked using;
  Blocked - see

Spamcop is a spam diagnosis service to which people can send their spam, it tries to figure out where the spam came from and sends off complaint messages. They're the largest user of the contact database, and they certainly know how works, so what was it doing on their blacklist?

Spamcop's blacklist is infamously hair-trigger. with the goal being to block spam as quickly as possible, even at the cost of blocking a fair amount of legitimate mail. (Their usual response to complaints about overbroad blocking is that their users like it the way it is. I don't use their blacklist, but there's no accounting for taste.)

I sent off a few tartly worded messages to Spamcop's management, they sent back a few responses along the lines of "oops" but with more colorful language, and I got un-listed. But what happened?

Spamcop feeds its blacklist with mail from spamtraps, addresses that should never get any legitimate mail. with some of the spamtraps being entire domains. Evidently sent mail to one of those spamtraps. I presume that what happened is that the spamtrap domain was forged in a spam message, someone (probably not using Spamcop) didn't realize it was forged and sent a complaint to that domain through, which was then forwarded to postmaster@spamtrap-domain, and blammo. Spamcop says that administrative addresses like postmaster are supposed to be excluded from the automatic blacklisting, but weren't. Oops, indeed. Now it's supposed to be fixed.

The message here is not that all blacklists stink, because they don't. It's that running a good blacklist is really hard, for reasons that are mostly not very technical. The best mechanically run blacklist I know is the CBL which is fed by spamtraps but does some careful analysis of that mail and only adds blacklist entries when the mail appears to have been sent by a virus or worm. Even the CBL has false positives, typically when a virus controlled machine and a real mail host are both behind the same firewall and share an IP address, although its error rate is rather low.

If computers weren't so vulnerable to being taken over by bad guys, the need for blacklists would be much less, but they are, so we're stuck with them.

  posted at: 01:15 :: permanent link to this entry :: 0 comments
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My other sites

Who is this guy?

Airline ticket info

Taughannock Networks

Other blogs

It turns out you don’t need a license to hunt for spam.
62 days ago

A keen grasp of the obvious
Italian Apple Cake
620 days ago

Related sites

Coalition Against Unsolicited Commercial E-mail

Network Abuse Clearinghouse

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