Here's a quick note about the DPM issue: "Backup administrators who've scheduled System Center Data Protection Manager (DPM) backups for Microsoft Exchange databases might run into backup failures if "circular logging" is turned on for those Exchange databases. In such cases, DPM incremental backups fails with VSS error 0x800423f4 and a retry of the backup job doesn't resolve the problem. This's expected behavior because any backup application willn't be able to perform reliable incremental backups for simple reason that the log gets automatically overwritten once Exchange app reaches the end of the log. If the Exchange administrator follows the recommended guideline of turning off the circular logging flag on the Exchange databases then backups can be reliable."
More Info: Exchange Circular Logging and VSS Backups