LitigationHoldEnabled Option

LitigationHoldEnabled - The LitigationHoldEnabled parameter specifies that the mailbox is under litigation hold and that messages can't be deleted from the user's account. The two possible values for this parameter are $true or $false. The default value is $false. After a mailbox is placed on litigation hold, deleted items and all versions of changed items are retained in the Recoverable Items folder. Items that are purged from the dumpster are also retained and the items are held indefinitely. If you enable litigation hold, single-item recovery quotas aren't applied

Set-Mailbox $User -LitigationHoldEnabled $true / $false