How to correlate ESP logs with Postmaster data?

Your ESP logs show what you sent and what status came back. Google Postmaster Tools and Microsoft SNDS show what the mailbox providers actually saw on their end. Neither source alone gives you the full picture. When you correlate them, you start to see the real story.

So Start with timing. If your ESP logs show a surge in deferrals or soft bounces on a specific date, check Postmaster for that same date. Did your domain reputation score drop? Did your spam rate tick up? A correlation in timing between what your logs recorded and what Postmaster shows strongly suggests a causal relationship, either your sending behavior caused the reputation shift, or an external factor (like a blocklist listing) caused both your delivery problems and the reputation drop simultaneously.

Look at authentication pass rates in Postmaster against your ESP's authentication reporting. If Postmaster shows SPF failing for a meaningful percentage of your Gmail-destined mail, but your ESP reports all authentication as passing, that's a discrepancy worth investigating. It often means messages are being forwarded, relayed, or sent through a path your SPF record doesn't authorize. For Microsoft-bound mail, compare SNDS IP reputation color codes against your Outlook delivery rates in ESP logs. A yellow or red SNDS status should correlate with elevated deferrals or blocks to Microsoft addresses. If they're not correlating, check whether you have multiple sending IPs and only some are in trouble.

The practical workflow: when something goes wrong with deliverability, open your ESP logs and your Postmaster/SNDS data side by side and look for where the signals diverge. Each disagreement is a question to answer. That investigative process, more than any single metric, is what experienced deliverability troubleshooters actually do.