How are listings propagated between services?

Blocklist data propagates through the email ecosystem in several ways. Understanding propagation helps you assess the urgency of addressing listings.

Most blocklists are queried in real time via DNS. When a mail server checks an IP, it queries the blocklist directly. There is no sync delay; listings take effect immediately.

Some services aggregate multiple blocklists into composite scores. These aggregators query several lists and combine results. A listing on one contributing list affects the composite assessment.

Commercial security platforms often incorporate blocklist data into their proprietary reputation systems. A Spamhaus listing influences how Proofpoint or Mimecast treats your mail, even beyond the direct blocklist query.

Internal ISP reputation systems may not directly use external blocklists but notice when your mail gets blocked elsewhere. This indirect effect can influence their own assessments.

News travels fast between harbors. A problem at one port soon affects your welcome at others.