The regular workflows for deleted mailbox’s in o365 are as follows, when a user permanently deletes their mailbox item from either their folders or the deleted items folder it is sent to a deletion folder within the recoverable items, after the retention period has expired the item is then purged.
A way to combat this retention period is enabling o365 litigation hold, this preserves the item even if the user deletes the item from their mailbox , if the users archive mailbox is also enabled the data is also placed on hold. This feature allows users data to be preserved meaning it can be used for e-discovery purposes, the time length of the litigation hold can also be configured. At the end of the selected period the data will then follow the default retention policy.
I recently ran into an issue with this when rehiring a users account, as the hold stops the remote mailbox’s from then being disabled which is a crucial step when changing a account in AD. It took me a while to find a solution for this problem but finally managed to resolve it below is the guide to do so.
1) Launch Exchange Management Shell
2) Set-ADServerSettings -ViewEntireForest $True
3) Disable-RemoteMailbox ‘User’s Address’ -IgnoreLegalHold
4) Run a AD Sync cycle
I hope this helps!