I’ll explain In-place Hold feature on eDiscovery, this commented by Office 365 for IT Pros which I bought.
In-place Hold : This defines how Exchange Online will hold content located by the search criteria. If set, you can hold data indefinitely or for a defined number of days based on the date the item was received or created in the mailbox.
All eDiscovery searches automatically examine deleted items held in the Recoverable Items structure, including those retained to meet in-place or legal holds imposed by this search or other searches. If an item is found in Recoverable items, it can be retrieved when the search is subsequently used to copy items to the discovery mailbox.
When I see that explanation , I see eDiscovery is not a unique place to store all messages, etc , this feature is related user’s hidden Recoverable Items structure, and you must know if Recoverable Items structure exceeds 100gb , deleted items shouldn’t work , so as to use effectively user mailbox under office365 environment, it’s better idea to set online archive to all user mailboxes