What is a blocklist and which ones affect cold outreach most?

A blocklist (or blacklist) is a database of IP addresses or domains identified as sending spam or unwanted mail. Mailbox providers consult these lists when deciding whether to accept messages.

Major blocklists affecting cold email:

Spamhaus: Most influential. SBL (spam sources), XBL (exploited hosts), DBL (domain blocklist). A Spamhaus listing causes widespread delivery failures.

Barracuda (BRBL): Widely used by corporate email systems. Common with B2B recipients.

Spamcop: User-reported spam database. Listings often temporary but affect delivery.

Invaluement: Focuses on snowshoe spam patterns. Relevant for multi-domain cold operations.

Microsoft SNDS: Not a blocklist but reputation data affecting Outlook/Microsoft delivery.

How blocklisting happens:

Spam complaints from recipients

Hitting spamtraps

High volume with poor engagement

Technical issues suggesting compromise

Prevention:

Quality list management

Reasonable volume

Quick response to bounce signals

Regular monitoring of blocklist status

Check your status regularly using tools like MXToolbox or MultiRBL.