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Deliverability

Blocklist

A list of IP addresses or domains that are blocked from sending email due to spam or abuse.

Definition

A blocklist (also called blacklist) is a database of IP addresses, domains, or email addresses that have been identified as sources of spam or abuse. Email servers consult these lists when receiving messages and may reject or filter emails from listed senders. Being on a blocklist can severely impact your email deliverability.

Why It Matters

Landing on a blocklist can devastate your email deliverability overnight. Major blocklists are checked by millions of mail servers worldwide. Even being on a single blocklist can cause significant delivery failures to many recipients.

How It Works

Organizations maintain blocklists based on spam reports, honeypots (fake addresses that only receive spam), and automated detection. When a mail server receives an email, it queries one or more blocklists. If the sender's IP or domain is listed, the email may be rejected or sent to spam. Each blocklist has its own criteria and removal process.

Best Practices

  • 1Regularly check major blocklists (Spamhaus, Barracuda, Spamcop)
  • 2Maintain good sending practices to avoid getting listed
  • 3Act immediately if listed to identify and fix the cause
  • 4Document your removal request with evidence of corrective action

Frequently Asked Questions

Use tools like MXToolbox, MultiRBL, or your ESP's reputation monitoring. These check your IP and domain against dozens of major blocklists and report any listings.

Each blocklist has its own removal process. Generally: identify what caused the listing, fix the underlying issue, then submit a removal request through the blocklist's website. Some lists remove automatically after the offending behavior stops.

The most impactful blocklists include Spamhaus (SBL, XBL, PBL), Barracuda, Spamcop, SORBS, and Microsoft's blocklist. Being on Spamhaus is particularly damaging as it is widely used.