More of a note about an effect I experienced using the new Microsoft Purview Compliance tool for Data Lifecycle Management and Retention Policies.

We are new to M365, and are still migrating to it. As part of the setup before I started migrating, I did create a “Default Retention Policy” in Microsoft Purview for Exchange objects. I started migrating, setup Veeam M365 Backup to protect the migrated mailboxes, and all was good.

As we expanded our use of M365, I reviewed the “Default Retention Policy” and learned a few more things about it. Long and short of the story, the default policy I had created was not applying to anything (for reasons undetermined). I deleted the retention policy and recreated it and all heck broke lose as the Purview retention policy applied.

At this stage of our migration, roughly 1 TB of mail had been migrated, and our “every 30 minute” Veeam snapshots were keeping up easily.

After the Default Retention Policy started applying to all of the migrated mail, the next scheduled snapshot turned into a “Full Backup” (for lack of better words) and had to backup all 1TB of migrated data as if it was brand new. The retention tag was enough of a change that Veeam saw every single item already backed up as new/changed.

I cannot even fathom what this would have done to my backups if I had a much larger environment to protect than 1 TB. My resources would have been overwhelmed, and backups would never complete.

If you already knew this was an issue, just a reminder. If you did not, and are looking at reviewing Purview and making changes to Retention Policies, keep it in mind. You might improve your retention policies to better suit your environment and blow-up your backups at the same time.