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Interpreting an SMTP protocol log

What the messages mean.

In a previous article I discussed using SMTP logging to troubleshoot communication issues. When the diagnostics...

logging level of the SMTP protocol log category is set to maximum the complete incoming message transfer is written to the log. To ensure that SMTP logging is enabled check the application event log after the Internet Mail Service is started. If enabled event 2004 is written to the event log. One point to note is that headers and body of outgoing messages cannot be logged. Only the entire text of incoming messages is included.

The following are transmission codes included in the logs:
2xx or 3xx message code indicate normal transmission of message
4xx codes are temporary problems that can be resolved by resending the message.
5xx codes are permanent negative responses requiring some repair before another attempt to send. Responses with the 4xx and 5xx codes are accompanied by text describing the problem.

The following is an example of a log text found in the SMTP log file:

4/2/01 11:44:31 AM : A connection was accepted from
4/2/01 11:44:31 AM : <<< IO: |HELO
4/2/01 11:44:31 AM : <<< HELO
4/2/01 11:44:31 AM : >>> 250 OK
4/2/01 11:44:31 AM : <<< IO: |MAIL  From:<>
4/2/01 11:44:31 AM : <<< MAIL From:<>
4/2/01 11:44:31 AM : >>> 250 OK - mail from <>
4/2/01 11:44:31 AM : <<< IO: |RCPT To:<>
4/2/01 11:44:31 AM : <<< RCPT To:<>
4/2/01 11:44:32 AM : >>> 250 OK - Recipient <>
4/2/01 11:44:32 AM : <<< IO: |DATA

For additional information on SMTP logging including a detailed log file visit the following link:;EN-US;q155455.

Adesh Rampat has 10 years experience with network and IT administration. He is a member of the Association Of Internet Professionals, the Institute For Network Professionals, and the International Webmasters Association. He has also lectured extensively on a variety of topics.

This was last published in May 2002

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