A spike in Spamhaus lists: that occurred

On June 7th at 6:00 a.m. PST, Twilio SendGrid’s automated systems alerted us to a significant increase in Spamhaus entries. Spamhaus is the best known and most powerful email blocklist provider in the email industry. A Spamhaus entry usually occurs when Spamhaus detects a significant amount of unsolicited email from an IP address and then adds that IP address to a list of known spam-emitting IP addresses.

However, this surge in entries affected hundreds of our customers’ IP addresses and some customer emails may have been blocked during this period. When examining Spamhaus entries, other email service providers also reported an unprecedented increase in Spamhaus entries.

This made it clear that the entries were not the result of increased spam or malicious mailings from Twilio SendGrid customers, and not just SendGrid customers.

Spamhaus has not commented publicly on the incident, but has confirmed that the increase in entries was due to a change in their system that resulted in IPs being mistakenly added to their blacklist. Spamhaus quickly rolled back the change and the problem was fixed almost immediately.

These excess entries were very short-lived and disappeared after just a few minutes, but there is still a good chance that portions of the senders’ messages were blocked during this period. Messages that are blocked between 6:00 and 9:00 a.m. PST and which Spamhaus stated as the reason for the blocking refer almost exclusively to these short-lived, incorrect Spamhaus entries and can be ignored in almost all cases.

This incident caused a brief interruption in e-mail delivery for many responsible and trustworthy senders. Nobody wants that. Least of all Spamhaus.

It is important to remember that Spamhaus plays an important role in protecting email as a viable form of communication.

Their track record as a reliable source for identifying malicious or unsolicited email communications is impeccable and we know they do not take such incidents lightly.

We will continue to monitor developments and do our best to keep our customers updated as we learn more.

Although this incident was an unexpected consequence of changes made by a blacklist provider, legitimate blacklists continue to emerge. For more information on blocklists and emailing best practices, see these resources: