Office 365 multiple email retention to support long email storage for your security services.
When you include a location such as a site or a mailbox in a retention policy, the content remains in its original location. People can continue to work with their documents or mail as if nothing’s changed. But if they edit or delete content that’s included in the policy, a copy of the content as it existed when you applied the policy is retained.
For SharePoint site collections, a copy of the original content is retained in the Preservation Hold library when users edit or delete it. For email and public folders, the copy is retained in the Recoverable Items folder. These secure locations and the retained content are not visible to most people. With a retention policy, people do not even need to know that their content is subject to the policy.