One of the main tools that any administrator can use is that of an Error Log. Application vendors supply them, OS administrators rely on them and most everyone has looked at one at some time or another.

For email, there are error and transaction logs that can contain much useful information about errors that occur during communications between Outlook email clients and various email servers such as Exchange server. This information can be very useful in diagnosing problems that have occurred with the different components of the email client. This article will provide some insight into what this error log contains and how it can benefit you.

Reviewing the diagnostic messages in the logs can be a very useful activity not only for identifying problems but also for spotting trends or patterns. Think of the process of reviewing the error logs as akin to listening to the sound of your automobile. Over time you learn which sounds are normal and which are not. Likewise, a regular review of the error logs can reveal, over time, which error messages are normal – due to lack of patches or fixes – and which errors are unusual and require further investigation.

E-mail server administrators can use some of the diagnostic data to troubleshoot problems with e-mail messages. Outlook allows for certain features to be enabled for logging and diagnostic purposes. Those features include the following:
- Calendar modifications
- Free/busy time changes
- Reminders
- Offline Address Book
- Transactions for the Messaging Application Programming Interface (MAPI), which is used with Microsoft Exchange Server, Post Office Protocol (POP3),  Internet Message Access Protocol (IMAP), Simple Mail Transfer Protocol (SMTP), and Lightweight Directory Access Protocol (LDAP).

The email logging option can be enabled with the following steps:

1. From the Tools menu, click Options.
2. From the Other tab, click Advanced Options.
3. Select the Enable mail logging (troubleshooting) check box and then click OK two times.
4. Restart Outlook.

After Outlook has been restarted you will notice the “Logging Enabled” will be displayed in the title bar. This message is a notification that the email logging feature has been enabled and all communications between the client and the server will now be logged. Any messages that are sent and received can be tracked through the email logs. Additionally all calendar transactions will be written to the log file including any transactions that have been generated by Outlook features, any user actions, the Outlook object model and any transactions generated by the MAPI calendar.

Additionally the log file will also have information about the following items:
- Sniffer (Inbox auto-processing)
- Free/busy publishing
- Reminders
- Calendar item actions (creation, modification, or deletion)
- Resource booking

As is true with most applications if you leave the logging enabled for a long period of time it will continue to log errors and transactions and thus it will continue to grow in size. Over time I have encountered more than a couple system administrators who have forgotten to turn off logging. What can happen is that systems begin to run more slowly as available disk space decreases and file systems begin to run out of space.

After logging has been turned on the next step is reviewing the log files. On Windows, the MAPI (Exchange), POP3, and SMTP transports log file can be found in the following location:
C:Documents and Settingsuser nameLocal SettingsTempOpmlog.log

On older versions of Windows such as Windows 98 and Millennium Edition (ME) the log file can be found in: C:WindowsTempOpmlog.log.

The log files for the IMAP transport on Windows systems can be found on:

C:Documents and Settings(user name)Local SettingsTempOutlook Logging(name Of IMAP server)IMAP0.log, IMAP1.log, and so on.

On older versions of Windows such as Windows 98 and Windows Millennium Edition (ME) the log files can be found at:

C:WindowsTempOutlook Logging(name Of IMAP server)IMAP0.log, IMAP1.log, and so on.

If there is a known situation that is hindering operations and error logging is turned off then an administrator can turn on logging temporarily and attempt to reproduce the situation. Once logging has been enabled an administrator can reproduce the necessary sequence of operations to recreate the problem and then go to the error log(s) to review the error code(s) generated.

Lastly, to disable logging perform the following steps:

1. From the Tools menu, click Options.
2. From the Other tab, select Advanced Options.
3. Clear the Enable logging (troubleshooting) check box.
4. Click OK two times.