Retention period for backup in M365 Security Plus

When configuring a backup schedule, you need to specify the maximum amount of time a backup can be held in your repository. For example, if you have set a retention period of six months, it ensures that the backup held in your repository only holds information of all changes made in the preceding six months. When you configure a new backup schedule with a backup retention of six months, the following happens:

• When the backup job begins, M365 Security Plus will analyze the metadata of all your Exchange Online mailbox data.
• The retention period's starting point is set to the current date and time.
• The last modification date of each item is identified, and the age of the item is calculated by comparing the last modified time and the current date and time.
• The age is then compared with the value that you’ve set as the maximum retention period.
• If the age of an item exceeds the specified retention period, the item will not be backed up into the repository.
• If the age is less than the specified retention period, the item will be backed up.

Note that the first backup alone is a complete backup and all consecutive backups are incremental. This how incremental backups work: assume a backup schedule is created on January 1, 2021 to perform a backup every Sunday at 12am. The retention period is set to one year. The first time the schedule will run is on January 3, 2021 at 12am. January 3, 2021 will be set as the starting point of the retention period.

All mailbox items that have their last modified entry from January 3, 2020 to January 3, 2021 will be backed up, as the age of the modified item will be less than the maximum retention period. Any item that has the last modified date earlier than January 3, 2020 will not be backed up.

This retention policy is then applied to the backed-up data on the repository.

• After all items that satisfy the age criteria are backed up, M365 Security Plus checks for items that have exceeded the retention period.
• Items whose ages exceed the retention period will be removed from the backup repository.

Considering the same example as above, the next installment of the backup will occur on January 10, 2021 at 12am. With January 10, 2021 set as the starting point of the retention period, M365 Security Plus will scan the backed-up data to find all items whose last modified time are earlier than January 10th, 2020. All items that fit the criteria are then removed from the repository.