How to use SMTP logs for spam-filter diagnosis?

SMTP logs record the conversation between sending and receiving servers. Analyzing these logs reveals exactly what happened during delivery attempts.

Look for response codes: 2xx indicates success, 4xx indicates temporary failures requiring retry, 5xx indicates permanent rejection. The specific codes and accompanying messages explain why.

Examine rejection messages in the logs. They often include references to blocklists, content policies, or authentication failures. These messages are your primary diagnostic tool.

Track timing in the logs. Rejections during connection indicate IP-level blocking. Rejections after DATA indicate content-level filtering.

For ongoing diagnosis, compare successful and failed deliveries. Identify what differs between messages that pass and those that fail.

SMTP logs are the ship's detailed voyage record. Every exchange with every port is documented for review.