In order to reduce the size of your Inbox, you need to export (manually archive) to an Outlook for Mac Data File (.olm) on your Mac.
You can still access this data at any time, and you can even transfer individual items, or the entire Data File, to Outlook for Mac on another computer.
For information on how to import the data from a "Outlook for Mac Data File (.olm)", please refer to "Import email messages, contacts, and other items into Outlook for Mac" for details.
How to export items:
When you export to an Outlook for Mac Data File, you can choose to filter information by category or by item type. For example, if you want to transfer networking information from your work computer to your home computer, you could export only items that you've assigned to the Networking category. Similarly, you might want to export all your contacts on your home computer and then import them on your work computer. During the export process, you can choose to keep the exported items in Outlook, or to delete them after they are exported.
- On the File menu, click Export.
- Click Outlook for Mac Data File.
- Select an option to filter by category or item type. (To export all items in Outlook, click Items of the following types, and then select all the check boxes.)
- Click the right arrow to continue, and then follow the instructions. (The Outlook for Mac Data File uses the .olm extension.)
- In Outlook for Windows, you can setup "AutoArchive" to archive Outlook items by specific time automatically on a regular interval. This feature is not available on Outlook for Mac, but you can archive data automatically by setting up Time Machine.
- An Outlook for Mac Data File (.olm) does not contain account settings or your Outlook preferences.
- For more information, you can refer to "Export items to an archive file in Outlook for Mac"