Microsoft Outlook, in the offline (Cached Exchange) mode, saves all the Exchange mailbox data in a local file named OST file. The changes made by the user in the offline mode are later updated to the server mailbox when the Exchange connection is re-established. As the OST file resides on the user’s computer, it is unaffected by Exchange Server crashes and disasters, Exchange mailbox deletions, etc. So, Exchange administrators and end-users can use OST files to restore user mailbox data in the occasions of such eventualities.

Here are some situations when converting an OST file to PST becomes very helpful:
A few Exchange mailboxes get deleted from the server
Want to back up a few Exchange mailboxes
An Exchange user wants to rebuild his OST file, but the user has some unsaved data in the OST file
EDB files are inaccessible due to Exchange corruption, Server crash, etc. and users want their emails urgently

Method 1: Export mailbox as a PST file
Login to Outlook with the Exchange account that was used to create the OST files. And to export the OST mailboxes to PST, follow the simple procedure:

- Go to Files >Open & Export and click Import & Export.
- Select Export to a file; then click Next.
- Select the second option—Outlook Data File (.pst)—and click Next.
- Select the folder which you want to export and click Next.
- Browse the location where you want to save the new PST file and choose the options regarding the duplicate items.

Method 2: Use ‘Archiving’ to save OST emails to PST
By enabling the Auto Archive method in Outlook, you will be able to save the items of the OST file in a separate PST file. This method helps you to keep the size of your mailbox manageable. When the size of the mailbox grows and there are multiple messages that are older and you do not require them to be in the mailbox, then you can put them in the archive folder easily. Follow the simple steps to enable auto archiving in Outlook.

- Launch the Outlook application, go to File, and click Options.
- Go to Advanced and click Auto Archive Settings.
- Now select the number of days in which you want to run auto archiving. Select the folder to save the archived file; also, provide the archiving options and finally click OK.

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