Retention policy to remove older than 30 days items from the Deletd Items folder "doesn't work "

Recently I worked with this scenario and faced the following "issue":

After a hybrid Exchange migration, we needed to replicate the onprem retention policies. Basically, enforce a policy that removes all the messages older than 30 days from all the mailboxes Deleted Items folder.

That's something quite easy to configure, ( done thousand times ), and will not get deeper in this technical details.

The case is that, after the setup, we noticed that many old items, ( even from last year and older ), were still in the user's Deleted Items folder without any action from the policy.

Of course that I know that the Managed Folder Assistant, ( in charge to check those policies and apply any action if required ), runs automatically once por week +- and we can force it with the cmdlt:

Start-ManagedFolderAssistant <UserEmailAddress>

And in order to recalculate all retention tags and apply them to the required folders, we can run:

Start-ManagedFolderAssistant <UserEmailAddress> -FullCrawl

But even running both commands and waiting more than 48 hours nothing happend.
After further investigations and the help of a MS engineer we got the point:

The retention age is calculated based on "Date of delivery or creation unless the item was deleted from a folder that does not have an inherited or implicit retention tag.
If an item is in a folder that doesn't have an inherited or implicit retention tag applied, the item isn't processed by the MFA and therefore doesn't have a start date stamped by it. When the user deletes such an item, and the MFA processes it for the first time in the Deleted Items folder, it stamps the current date as the start date."

How retention age is calculated in Exchange Online | Microsoft Learn

In fact, we verified that with some random old messages in some mailboxes and all those had the first MFA run as the start date stamped. After waiting 30 days the messages were correctly deleted.

Hope that this helps someone facing the same behavior, avoiding spend so much time as I spent on this 🙂