One of the biggest spam block lists on the internet suffered an embarrassing technical cock-up today which blocked emails from some servers at web security monitoring firm MessageLabs and at some ISPs for about five hours.

The CBL (Composite Blocking List), a DNS blacklist that takes its source data from very large spamtraps and lists IP addresses of open proxies and worms/viruses, said on its website that a problem in a contributory feed spat out a number of invalid lists.

The rogue feed has now been removed and all IP addresses on the blacklist have been purged as a precautionary measure.

“Here at the CBL we take false positives extremely seriously, and we do apologise for this issue. We have taken steps to ensure this cannot happen again in future,” it said.

A spokeswoman at MessageLabs told The Register that others were affected by yesterday's false positive.

According to “word on the street”, Microsoft's Hotmail was also blocked from sending email from some of its servers for a large part of the day, she said.

MessageLabs said in an official statement:

“As soon as we were aware of the issue we worked with CBL to rectify the problem and I can confirm that our IPs have now been removed from the listing. We would recommend that any third parties ensure that their CBL list is up-to-date.”

The CBL has more about its rather embarrassing case of mistaken identity here. ®

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