An Outlook online archive mailbox can store older email messages where they won't occupy space in your primary mailbox. This specialized mailbox appears alongside your other mailbox folders in Outlook.
An administrator enables your archive mailbox and also sets the policies that control when emails will be moved to archive and how long they will be saved there for you. Additionally, you may be able to modify the standard policy settings to further manage the size and performance of your primary mailbox. And, if enabled, you can manually move or set rules to auto-move messages to the online archive.
Caution: When navigating mail storage locations, careful not to confuse the Online Archive folder with your existing Archive mail folder. For example, Move to Archive action in the ribbon, places the selected item into your local Archive, not the Online Archiv