How to diagnose placement issues by ISP?

ISP-specific diagnosis:

Segment your metrics: Break down opens, clicks, and bounces by recipient provider. Identify which ISP has the problem.

Provider tools: Check Gmail Postmaster for Gmail issues, SNDS for Microsoft issues.

Targeted seed tests: Run tests focused on the problem provider.

Provider-specific requirements: Review if you meet that provider's specific requirements (bulk sender rules, authentication, etc.).

History check: Did something change recently that correlates with problems at this specific provider?

When one port turns you away, investigate that port specifically.