We recently discovered that an Exchange Server could not accept any (external) messages anymore due to low disk space. It is the Exchange "Back Pressure" feature that caused the Exchange Server to stop sending emails.

Now using a tool I found that there are many log files (.log) in directory:

C:\Program Files\Microsoft\Exchange Server\V14\Mailbox\Mailbox Database <id>\

These log files take up several GBs and thus all my resources (disk space), causing my problem.

I had that specific directory open for a few minutes and it was updating and adding logs. To me it looks like it logs every incoming (?)/outgoing (?) email.


  1. Is it safe to manually delete these mailbox database .log files or should this be done by an Exchange tool?
  2. How can I control this logging? (enable/disable/auto-cleanup/... settings?)
  3. What exactly does such a mailbox database .log file contain?

Thanks in advance.

  1. No it is not safe to simply delete log files to free space. They are critical to the functionality of Exchange.
  2. You can use an Exchange aware backup solution that can truncate the logs (delete them) once a backup is completed. Another option is to use circular logging (But will remove recoverability of data if you do not have a backup solution). This is extremely high level and you need to decide on which strategy is best for your environment.
  3. Log files track changes to a users mailbox and to ensure no loss of data in the event of a dirty shut down of a database.

In addition, you've created a mailbox in the default location. It is best practise to create a new volume for the databases and another for the log transaction files.

Good article describing the same issue you have: http://exchangeserverpro.com/exchange-2010-faq-disk-filling-log-files/

  • Another idea to save your server if you know the database is in good state , you can use circular log to exchange database . So when someting is commit in exchange database the log remove information about this data in log database. But its not good for rpo backup (if you want to go to backup + 3 minutes) . – YuKYuK Feb 18 '15 at 10:59

My solution:

Backup the Server once (Windows Backup). In the options select Advanced and for VSS (Volume Shadow Services) select Full. This will backup the server and compact the log file.

The log is needed for recovery. So when you backup with VSS settings to Full, Windows Server will compact the log after a successfull backup. Because your data is backupped, it is sure it can compact the log because recoverey can be done via the backup instead of the transaction log file.


For me the backup solution didn't work out, although I have set up a daily incremental backup in Windows Backup, the logs were not truncated, as stated by Microsoft docs. So my solution was to enable circular logging:

Set-MailboxDatabase Your-Database-Name -CircularLoggingEnabled $True

and simply reboot the server. (you could also unmount and mount the mailbox datastore)

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