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Spamhaus CSS


The Spamhaus Composite Snowshoe (CSS) blocklist covers snowshoe spamming, risky sending, and compromised hosts. Snowshoe spamming in a technique designed to avoid spam filters. Like a snowshoe walking over snow, snowshoe spammers attempt to distribute spam across a broad set of IPs and domains to avoid detection.

CSS listings are primarily influenced by:

  • Unsolicited emails
  • Broad-spectrum aggregated views of email deliveries
  • Poor list-hygiene
  • Spam traffic due to a compromise (compromised account, webform or CMS)
  • Indication of low reputation or abuse


This listing happens at an IP level. Delivery has a high chance of being negatively impacted.

Mitigation process

To better understand the potential problems that caused the listing, review the CSS website. Once you have reviewed, you should then:

  1. Find and correct the root cause of the listing.
  2. Contact Mailgun and provide details on root cause and what was done to resolve. Mailgun will then start the removal request.

It is important that the issue be resolved prior to contacting Mailgun. Being honest and transparent will get you to a solution faster. Spamhaus is straightforward on what they want to know. If Spamhaus detects that a problem is ongoing they will not remove the block.