Successful backups of Exchange 2007 do not flush the Exchange transaction logs
This can be caused if the Exchange replication is in a failed state.
1. Check the application event logs on the Exchange server for any informational or error messages regarding the logs being flushed.
*Even though the logs aren't being flushed, the event logs can still show no errors like everything is normal.
Event ID: 9827
Description: Exchange VSS Writer (instance 1a11ecc8-5f29-4d2f-9834-3782d3d03651:73) has successfully completed the full or incremental backup of replicated storage group 'First Storage Group'. The log files will be truncated after they have been replayed
2. Check the status of the replication by using the Exchange Management Console or using the cmdlet 'get-storagegroupcopystatus' from the shell.
3. Examine the output (i.e. 'failed', 'disabled')
a. If replication is disabled on the storage group(s), this article does not apply
b. If the replication status shows failed, this is most likely issue.
At this point, there is an issue with the Exchange replication and Microsoft may need to be contacted to help resolve the issue.
As a temporary workaround while investigating the replication issue, disabling replication and re-running the backup will properly flush the transaction logs.
*This method should only be used if disk space is considered critically low (Resolving the replication issue should be top priority)
Exchange Backup Method is set to FULL OR INCREMENTAL with the note "Flush committed logs"