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.
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