Exchange restore backup. What happens to local ost files?

A virtual hard disk crashed containing our sql server and exchange server. I have several backups but the one I can use is 3weeks old. What happens to local Outlook clients when I restore that Hyper-v backup? Will all email from the clients from the last 3 weeks by deleted during sync?

Solution 1:

Interesting question. I've never dealt with it directly (restoring that far back) to care what happens to the OST files, so I found this article online: http://searchexchange.techtarget.com/answer/What-happens-to-ost-files-after-restoring-an-Exchange-backup

To sum up what Brien thinks would happen:

The contents on the Exchange server will be considered the correct versions of their mailbox and authoritative.

SO, it will end up removing the last 3 weeks email in their OST file and making the OST files look like what is on Exchange.

However, I'm not sure I agree with him on this one...since his test is removing email in OWA and the server is still online. So the server KNOWS that the email was deleted.

I tend to agree with this here: http://social.technet.microsoft.com/Forums/office/en-US/89be9383-6efa-400e-9978-db13b88313f6/what-happens-to-ost-files-when-an-old-exchange-backup-is-restored?forum=exchangesvravailabilityandisasterrecoverylegacy

where the OP themselves stated:

after the new mailbox store was created, the clients' Outlook OST files did not lose any data

There it seems more likely...that the OST file has emails that are NEWER than what Exchange knows about and then syncs them BACK to the Exchange mailbox store.

An example TEST for this would be easy enough.

1. Disconnect your computer from the network
2. Open a PST file in Outlook with a recent email from this week in it (if possible) and copy an email from it to your local OST's Inbox
3. Verify that the email is showing in your OST's Inbox
4. Connect your network cable again and sync your email
5. The email you added to your OST from your PST should sync back to Exchange (verified via OWA)

BTW, I just did this test myself and it works...

HOWEVER, IF you are the least bit skeptical, I would create an exported PST file on each user's Outlook first of their OST file (at a minimum grabbing the last 3 weeks worth but easier would be the entire OST). This I would probably do regardless for anyone that is a VIP...better safe than sorry with them.

BUT TO SUM UP MY ANSWER:

In my test I show that the email in the OST file should be there. I would still recommend backing up the local OST files to PST's first if you have the resources to do so. The main reason I state this is because I'm not certain the existing OST file will be the one used by Outlook after the restore completes or if the sync will suddenly cause the server to want to create a new one. If it does you technically won't lose the old OST file, but you would need 3rd party tools at that point to extract email from them or convert them to PST.