You might have stored Outlook items in an Outlook Data File (.pst) that you created in classic Outlook. If you want to open or close a .pst file, you now can do that in a few simple steps from either new Outlook or classic Outlook.
If your online mailbox is nearly full, you can export some items to an Outlook Data File (.pst). You can also use these files to create archives, project files, or backups from any Outlook account.
To transfer messages and other items from a Windows-based computer to a Mac, Outlook for Mac can import Outlook Data Files (.pst) that were created in Outlook for Windows.
If you need to set up Outlook on a new computer, copy your .pst file from the old computer, then set up your email account on your new computer. You can then open the .pst file from the new computer.
To learn how to import items after you've exported them, see Import email, contacts, and calendar from an Outlook .pst file. Note: Importing a mailbox or calendar using a .pst file is currently not supported in new Outlook at this time.
Outlook Data Files (.ost) are used when you have an Exchange account and want to work offline or use or use the default Cached Exchange Mode. This type of data file is also used for accounts that you set up with the Outlook Connector for Outlook.com (formerly Hotmail).
If you want to back up the message folders locally (in addition to keeping them on the server), you can automatically move or delete older items with AutoArchive or export the items to a .pst file that you can restore later as needed and use by importing.
