In Exchange 2010, you can place a mailbox on retention hold or legal hold. Both holds serve a different purpose. It’s important to understand the functionality provided by both.
Retention Hold: As the Messaging Records Management page in the EMC suggests, retention hold is used to halt retention policy, which means the Managed Folder Assistant (MFA) does not move or delete items from the mailbox. It’s typically used when a user may not have mailbox access for an extended duration, for example, when the user’s on vacation. You can also use retention hold during the initial phase of MRM deployment and remove it as users become more comfortable with MRM functionality.
Should users access their mailbox when it’s on retention hold, they may notice that items aren’t being deleted or moved to archive. Mailbox size increases as more email accumulates and you may need to raise the user’s mailbox quotas to ensure mail flow to and from the mailbox isn’t interrupted.
The mailbox owner or any user (or process) with mailbox access can delete messages when the mailbox is on retention hold. Deleted messages are not treated any differently when a mailbox is on retention hold. Deleted items are retained until the deleted item retention period (14 days by default) and then deleted permanently. See the Retention Hold section in Understanding Retention Tags and Retention Policies for more details.
When placing a mailbox on retention hold, you can specify a start date and an end date. This allows you to configure retention hold in advance, and have it end at a predetermined date. Takeaway: retention hold is for planned activity, suspends move or delete actions taken by the MFA. If you haven’t deployed either MRM feature (Retention Tags or Managed Folders), retention hold is unnecessary.