How to read full SMTP transcripts for errors?

An SMTP transcript is a complete record of the conversation between your server and the receiving server. When that conversation fails, the transcript shows you exactly where it broke and why.

The conversation follows a predictable pattern. Your server opens with EHLO (it's saying "hello, here's what I can do"). The receiving server responds with capabilities. Your server then issues MAIL FROM (the sender address), RCPT TO (the recipient address), and DATA (the message content). Each step gets a code response. When everything works, you get 2xx codes (success). When it fails, you get 4xx (temporary) or 5xx (permanent) codes.

Reading a transcript is about finding where the codes change from good to bad. Look at each step. Did the connection open? Did EHLO succeed? Did the server accept your MAIL FROM? Where did it first reject or warn? That's your problem spot. The response text after the code explains the reason. "550 5.7.1 Authentication Failed" happens at the MAIL FROM stage and points to SPF, DKIM, or DMARC problems. "421 Service Not Available" at any stage usually means temporary network or server issues.

The transcript also tells you which server rejected you. Look at the hostname in the responses. Different servers might have different policies. Gmail might accept your mail while Yahoo rejects it. That's provider-specific filtering or reputation checking, not a universal problem.

Most ESPs make transcripts available in their bounce detail view or error logs. Some show them inline in the delivery report. If you're debugging, request the full SMTP conversation from your provider rather than just the bounce code.

Next step? Grab your most recent bounce, find its SMTP transcript in your ESP, and follow the conversation step by step. The first 5xx code you see is your actual problem.