How to identify which blocklist caused the delivery issue?

Identifying the responsible blocklist requires analyzing available data: bounce messages, delivery logs, and comprehensive blocklist checks.

Bounce messages often identify the blocklist directly. A rejection message might say "blocked by Spamhaus" or include a URL to the listing lookup. Read the full bounce text carefully.

If bounces are unclear, use multi-list checking tools like MXToolbox or MultiRBL to scan your sending IPs and domains against all major lists. This reveals any current listings.

Check both IP addresses and domains. Some issues are IP-based (Spamhaus SBL) while others are domain-based (Spamhaus DBL). Also check any URLs in your message content against URI blocklists.

If no public blocklist appears, the problem may be an internal provider list that you cannot directly query.

Follow the clues in the rejection notice. Most harbors tell you why they turned you away if you read the message carefully.