We have recently changed the way, removing an Office 365 user with attached mailbox, behaves.
If we deleted an active user while he was still licensed, his mailbox would go into a “soft deleted” state and could have been recovered, just by restoring the user.
If we also purged the MsolUser from Office 365 with “Remove-MsolUser -ObjectID <guid of soft deleted user> -RemoveFromRecycleBin -Force”, his soft deleted mailbox would also be purged, unless it was specifically put on Litigation Hold or In-Place Hold.
The first situation remains the same, as in if we soft delete the user account, while he still has a license assigned, his mailbox will go into a soft deleted state.
The change however, is that if we purge the user account, the mailbox is no longer purged with it, instead it will remain in a soft deleted state, regardless if previously it was put on Litigation Hold/In-Place hold or not.
Read more at source: https://blogs.technet.microsoft.com/exovoice/2016/05/24/how-to-purge-a-soft-deleted-mailbox-in-office-365/