Skip to main content

Cisco spam filter mistakenly blocked thousands of legitimate emails

(Image credit: Image Credit: Evannovostro / Shutterstock)

An expired Cisco domain has caused thousands of legitimate emails to be erroneously labelled as spam, preventing them from leaving the outbox.

According to a new report from Bleeping Computer, the issue relates to Cisco's email spam filter, SpamCop, which provides a Real-time Blackhole List (RBL) that mail servers use to determine if incoming mail is spam or not.

As the domain had expired, mail servers that used Cisco’s RBL started rejecting incoming mail automatically. Thousands of organizations, ISPs and mail administrators all over the world found outgoing emails listed as spam in the blacklist at

Cisco was quick to remedy the issue by renewing the offending domain, but some mail administrators are still having issues.

When an email is blocked, administrators see the following error in their mail server logs:

"The error on is: An error occurred while processing your request."

These errors could be due to cached DNS lookup in local DNS severs. Once the DNS TTL expires on the domain or admins flush the cache, the RBL should function normally.