A Mailbox is full and reached its limit. The user is trying to empty out “Deleted Items” in Outlook, and it throws “You can’t permanently delete these items”.
And, the user goes to “Recoverable Items” and tries to purge items in Outlook. They couldn’t purge items either. Now what?
It happens because SingleItemRecoveryEnabled for the user’s mailbox is set to TRUE (default value).
What is Single Item Recovery? Microsoft document explains
If single item recovery is enabled, messages that are purged (hard-deleted) by the user are retained in the Recoverable Items folder of the mailbox until the deleted item retention period expires. This lets an administrator recover messages purged by the user before the deleted item retention period expires.
https://docs.microsoft.com/en-us/Exchange/recipients/user-mailboxes/single-item-recovery?view=exchserver-2019
In that explanation, mentions Deleted Item Retention Period which is the mailbox property called RetainDeletedItemsFor.
So Exchange is waiting for the RetainDeletedItemsFor time period to expire, so it can delete those emails. Till then, those emails should stay in Recoverable Items.
Obviously, the resolution is to set SingleItemRecoveryEnabled to false and set RetainDeletedItemsFor to some lower value (min: 0 and Max: 24,855 days). I wouldn’t suggest this usually. Since the mailbox is full (100 GB in Office 365) this is an extreme situation, so we need extreme measures.
Here are Exchange PowerShell commands to accomplish the above solution:
Connect to Exchange Online or Exchange OnPrem server in PowerShell
E:\> Set-Mailbox <Mailbox Name> -RetainDeletedItemsFor 1.00:00:00
E:\> Set-Mailbox <Mailbox Name> -SingleItemRecoveryEnabled:$false
WARNING: The single item recovery setting may take up to 240 minutes to take effect.
E:\> Start-ManagedFolderAssistant <Mailbox Name>We are running Start-ManagedFolderAssistant to take effect on the settings right away. I would wait a few minutes. Inform user to purge items from Recoverable Items, Empty out Deleted Items, and Deleted some old/unneeded emails.
P.S. I would change the mailbox property values to default after a day or two.
I hope it helped you. Leave me a comment.
Anand, the Architect 
Works like a charm..
our printer solution reads a lot of emails, and deletes them after fetching attachments. After some time, when it is extremely busy with students printing mails cannot be deleted, and will be processed over and over, resulting in the printserver overheating, and spool directory filling up.
I have had Microsoft look in to this, but they could not find a cause…
Hi Anand,
thank you very much for your solution! It works perfectly. I wonder how to change the mailbox property values to default?
Hello restorweb,
Here is the values and command to set back the mailbox to default values:
E:\> Set-Mailbox -RetainDeletedItemsFor 14.00:00:00
E:\> Set-Mailbox -SingleItemRecoveryEnabled:$true