When Retention tag is applied to Deleted items folder.you would see a different behavior in which Retention tag procesess other folders and the Deleted Items folder.The retention age of mailbox items is calculated from the date of delivery,but when it comes to Deleted items folder,it works differently.
The retention age for items in the Deleted Items folder is calculated based on the date of delivery unless the item was moved or deleted from a folder that doesn't have an inherited or implicit retention tag.
The behaviour is explain in Technet article below;
How Retention Age is Calculated.
Example 1 :
• A user receives a message in the Inbox folder on 01/26/2011. The Inbox folder has a retention tag configured to delete items in 365 days.
1. The Managed Folder Assistant processes the message. It stamps the message with a start date of 01/26/2011 and an expiration date of 01/26/2012.
2. The user soft-deletes the item on 02/27/2011.The item is moved to the Deleted Items folder, which has a retention tag configured to delete the item in 30 days.
3. The Managed Folder Assistant processes the message. It recalculates the expiration date based on the start date (01/26/2011).
4. Because the item is older than 30 days, it is deleted immediately.
• A user receives an item in the Inbox folder on 01/26/2011. The Inbox folder doesn't have a retention tag applied, and the retention policy doesn't contain a default policy tag.
1. The user soft-deletes the item on 02/27/2011. The item is moved to the Deleted Items folder, which has a retention tag configured to delete items in 30 days.
2. The Managed Folder Assistant processes the mailbox on 03/27/2011, and determines that the item doesn't have a start date. It stamps the current date as the start date and 04/27/2011 as the expiration date.
3. The item is deleted on 04/27/2011, which is 30 days after it was deleted or moved to the Deleted Items folder.
In above Example 2,when the message is soft-deleted on 02/26/2011,the message is moved to Deleted Items folder,in above example we see that Folder assistant process the mailbox on 03/27/2011.The question can raise is why is assistant is processing the message after one month .
This is just an example to show the Deleted Items behaviour,Ideally the message in the Deleted Item folder or any message in that matter would be processed quite faster,this depends on the ManagedFolderWorkCycle and ManagedFolderWorkCycle checkpoint.
Because of the Deleted Items folder processing behaviours,commonly faced issues are;
1) Outlook in cache mode shows the message is already expired,but Retention policy is not deleting this expired message or moving it to archive. This is because Outlook in cached mode pulls the Policy Tag information for the folder and then does the calculation for expiration itself. But if you see the same message in OWA,you would find that the message is not
yet expired. This usually happens with items in Deleted Items folder,because the way the Deleted Items folder is processed.
2) Another common issue is, where Retention policy is implemented because users have lot of items in Deleted Items folder and administrator is implementing Retention policies on Deleted Items for the first time.Therefore when the Deleted Items folder is processed for the first time,items are stamped with a Start date which is date when message was first processed,thus messages in the Deleted Items are going to be deleted after a delay.
For example,users have 1 year old emails tag on the Deleted Items folder When MFA processes items in deleted Items folder for the first time,it would not see a start date on these items so it will stamp a start date as today
and expiry date would be the date when message was first processed plus 1 year.