How to analyze delivery logs?

Your sending platform is generating detailed logs of every delivery attempt, and most senders barely look at them. When something goes wrong, those logs are often the fastest path to understanding why. Here's how to actually use them.

So Start by understanding what each log entry contains: the recipient address, the sending timestamp, the final status (delivered, bounced, deferred, or failed), the SMTP response code, and the raw response message from the receiving server. That response message is the most useful field when you're troubleshooting. It often contains a specific reason: "User unknown," "Mailbox over quota," "Message rejected due to spam content," or a reference to a blocklist. Read it.

For day-to-day analysis, you're looking for patterns rather than individual messages. Pull your bounce codes and group them. Are you seeing lots of 550 "user unknown" errors? That suggests list hygiene problems, addresses that never existed or have since been deactivated. Lots of 421 or 451 deferrals from a specific provider? That's a rate limit issue. Sporadic 550 errors mixed with successful delivery to the same domain? Possibly a sender reputation problem affecting a subset of your mail.

Segment your log analysis by recipient domain. Gmail, Outlook, Yahoo, and corporate domains each behave differently. A problem showing up at Outlook but not Gmail tells you something very specific about where to look. Most ESP dashboards let you filter by domain. If you're on a self-hosted setup, you'll need to parse logs manually or with a tool like grep, awk, or a purpose-built log aggregator.

When something urgent comes up, our free Email Header Analyzer can help you trace individual messages and identify exactly where they got stuck or rejected.