How to confirm if a blocklist or ISP-specific penalty is in effect?

Confirming blocklist or ISP penalties:

Blocklist checks:

MXToolbox blacklist check (multi-list)

Spamhaus direct lookup

Barracuda lookup

Check both IP and domain

ISP-specific indicators:

Bounce messages citing specific policies

Postmaster tool warnings

Provider-specific filter results

Testing:

Send to seed addresses at affected ISPs

Compare results across providers

Check if problem is isolated or widespread

Error message analysis:

Look for blocklist names in bounce text

Look for policy or reputation references

Note any remediation URLs provided

Check the banned ships registry. Contact port authorities for specific restrictions.