This content is part of the Essential Guide: The essential guide to Exchange Server backup and recovery

What happens to .ost files after restoring an Exchange backup?

An admin is curious as to what will happen to users' .ost files after restoring a backup. Our expert explains how to find out with a few quick steps.

I'm considering performing a point-in-time restoration of an Exchange mailbox database. Since this action rolls...

the database back to an earlier point in time, I'm curious as to what happens to the .ost files on my users' workstations. Can you please advise?

If you're curious about what will happen to the .ost files, all you really need is a copy of Outlook, Outlook Web App (OWA), some junk mail you can delete and the following steps.

To start, open Outlook and let the latest mail synchronize from the Exchange server to Outlook's .ost file. When the synchronization completes, unplug the network cable from the computer. Next, use another computer and log on to the same mailbox from OWA and delete a message or two. After doing so, log out of OWA.


You can also make a backup copy of the messages directly from Outlook if you like.

At this point, you've created an inconsistency. The recently deleted messages have been removed from Exchange, but they still exist in Outlook's .ost file.

When you plug the network cable back into the computer and open Outlook, the changes to the mailbox database will be replicated to the .ost file. This includes deleting messages that presently exist in the .ost file, but no longer exist within the mailbox database. This occurs because Outlook considers the contents of the Exchange Server mailbox to be authoritative.

The Exchange server's contents will always take precedence over anything stored in an Outlook .ost file when synchronization occurs.

About the author:
Brien Posey is an eight-time Microsoft MVP with two decades of IT experience. Before becoming a freelance technical writer, Brien worked as a CIO at a national chain of hospitals and health care facilities. He has also served as a network administrator for some of the nation's largest insurance companies and for the Department of Defense at Fort Knox.

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