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How to analyze Postfix mail logs

Do you use Postfix for your mail processing? The Postfix engine will send logging information to the maillog file, typically located in the /var/log directory. Understanding how to read the logs will help if problems ever arise. This process is great for analyzing the mail queues as well as digging for details when mail service isn't working as expected.

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Before understanding the records in the log file, it is important to know that you can configure the amount of information logged. By default, the logging level is set of normal. You can modify the master.cf file to increase the logigng level. Adding -v, -vv, or -vvv to the startup command will increase the logging level to verbose, very verbose, and very very verbose, respectively. If your mail server is heavily used, you won't want to leave these configuration options in place for long without fear of using up enormous amounts of disk space.

There are four daemons, and thus four record formats, you'll find in the file:

  • postfix/qmgr
  • postfix/smtp
  • postfix/smtpd
  • postfix/cleanup

postfix/qmgr
The postfix/qmgr records represent messages moving within Postfix's internal queues. If removed is part of the record, the log record contains when a message was removed from the mail queue. The other type of record includes from, size, and nrcpt information, where nrcpt stands for number of recipients.

Jul 2 12:24:48 username postfix/qmgr[12345]: 1234567890A: removed Jul 2 12:24:48 username postfix/qmgr[12345]: 1234567890A: from=user@example.com, size=1234, nrcpt=1 (queue active)

postfix/smtp
The postfix/smtp entries record information related to mail delivery. The relay attribute represents if a message was inbound or outbound. A relay value of localhost or 127.0.0.1 means the message was inbound as it identifies the MTA server that received the message. The other important bits of information are the identity of the sender and recipient. The delay field indicates the total amount of time Postfix spent processing the message, whereas delays is the time each daemon spent processing the message. There are four values for delays, in order they represent the smtpd, cleanup, qmgr, and smtp daemons.

Jul 2 12:24:48 username postfix/smtp[12345]: 1234567890A: to=user@example.com, relay=198.51.100.0[198.51.100.0]:25, delay=5.3, delays=5.1/0.02/0.02/0.2, dsn=1.2.3, status=sent (250 1.2.3 <20140702012658.1234567890A@hostname.example.com> Queued mail for delivery)

postfix/smtpd
Your postfix/smtpd records identify the host or ip address that connected to the SMTP daemon. For outgoing mail, the host specified will be localhost or 127.0.0.1, With inbound mail, it is the connected client.

Jul 2 12:24:48 username postfix/smtpd[12345]: connect from unknown[127.0.0.1] Jul 2 12:24:48 username postfix/smtpd[12345]: 1234567890A: client=unknown[198.51.100.0]

postfix/cleanup
The final daemon record is for postfix/cleanup. Here you'll just find the identity of the message just processed. 1234567890A represents message id and is also used as the temporary filename while the message is being processed.

Jul 2 12:24:48 username postfix/cleanup[12345]: 1234567890A: message-id=<20140702012658.1234567890A@hostname.example.com>

Searching the log file
Creating regular expressions and grepping the log file will allow you to analyze your Postfix mail logs and pull out detailed information. What you do with it after extracting is totally up to you. For example, you could consider adding the records to a database for further analysis or graphing the information to perhaps show peak usage times.

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