How do you identify which layer blocked you (ESP, ISP, filter, recipient)?

Identifying which layer blocked your message requires analyzing available data: bounce messages, logs, and delivery reports.

Bounce messages often identify the blocker. References to specific filters, blocklists, or policies indicate where rejection occurred. The rejecting server's hostname may identify whether it is a gateway, ISP, or final destination.

ESP delivery reports show where in the chain delivery stopped. Messages that failed at acceptance never left your ESP. Messages accepted but bounced later were rejected downstream.

Error code patterns help identify layers. Gateway filters often return specific error codes different from final destination mailbox providers.

If the rejection mentions Proofpoint, Mimecast, or similar services, a gateway filter blocked you. If it mentions Gmail or Microsoft policies, the final provider blocked you.

Read the rejection like a map. It shows where your journey ended and often why.

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