Recently my Client contacted me saying they cannot Receive External Email.
After checking the disk space i noticed that the drive had less than 15% available.
Upon investigation i found that DPM was not doing backups anymore and the log files had increased over a 3 week period on the Exchange Server.
The log files were deleted and when DPM was brought back online it gave an error that the log files were inconsistent.
My solution was to enable Circular logging in Exchange, Dismount / Mount the DB and then after replication i disabled Circular logging again. Once the DB was Dismounted / Mounted again i started the DPM backup and it went through again without any issues.
Hope that this helps anyone else.
Circular logging can be enabled through the EMC or Management Shell. Please note that if you have a DAG you do not need to Dismount/Mount the Database.
Shell Command: Set-MailboxDatabase -CircularloggingEnabled:$true