Spamhaus

Spamhaus operates the most widely used real-time blocklists (RBLs) on the internet, with SBL (Spamhaus Block List), CSS (Composite Blocking List), and other zones checked by thousands of mail servers worldwide. Their free lookup tool tells you instantly whether your IP or domain is listed on any Spamhaus blocklist, and explains the specific reason for listing. A Spamhaus entry is one of the fastest ways to completely tank your deliverability, so this check is essential.

If you're listed, Spamhaus provides a detailed delisting process depending on the list (some automatic, some requiring manual review). Major ISPs and corporate mail systems rely on Spamhaus, so clearing a listing here pays dividends immediately. Even if you're not listed, run a check monthly as part of routine deliverability maintenance, especially if you're sending high volumes or share infrastructure with other senders.