Note: This is not a Microsoft Exchange problem, but an Outlook 2013 one, and even though it might seem as a duplicate of a previously asked one, please pay attention to the end of the post.

A little background of my situation:

I have used Outlook 2010 for some time, and then upgraded to Outlook 2013 a few months back (had a severe synchronization problem with my IMAP server resulting in massive duplicating of emails, so I thought upgrading to 2013 will resolve it). (Spoiler: It didn't)

A few days back I had to reinstall my Windows 7. As part of saving my old data I also copied .ost (new Outlook 2013) data files to the external drive.

Now, after I've reinstalled W7, I wanted to restore them back. To my astonishment, I discovered that I can't import .ost data files back into Outlook 2010.

I didn't even make a distinction between the two formats up until then, so after I educated myself a little on the internet, I learned that .ost was (re)introduced as default data file format by Outlook 2013, supposedly bringing great improvements, new caching algorithm, reducing file size etc...). So I upgraded to 2013 again in hope of being able to use my emails again.

Now comes the catch. Outlook 2013 doesn't recognize it's own .ost file either (doesn't let me import it as data file), although it created a new .ost file and started using it without a problem.

So I thought ok, even though I can't import my old data file, maybe I can still trick Outlook into using it. But no matter what I've tried (creating new email accounts both from CPanel and Outlook hoping to be allowed to point in to my old data file, even changing every registry entry from its new data file to my old one), I had no success.

New connection parameters are absolutely the same as old ones (mail server, username & password, even hard drive locations and everything). But when I sneakily shutdown outlook, delete its new data file and replace it with my old one (desperate measures), he either renames it to .ost.old or just creates a new one with (1), (2) etc. Makes it even weirder.

After I've given up, I tried miscellaneous .ost recovery tools lying around the web, but none of them even remotely worked. After 3 hours of unsuccessful recovering (file size is 5.5 GB), one software reported it as "severely damaged", even though I'm positively sure that nothing is wrong neither with it, nor with it's two backups.

I've been through Google up and down and came up with nothing. This surprises me, because I can't be the only one having this kind of trouble (although Outlook 2013 is fairly new and Google results take some time to ease in).

File -> Import also doesn't work, since I don't have an option to choose .ost file, and forcing it results in file format not recognized error.

Lastly, I have looked through the following questions (and some more), and none of the answers were able to help me:

Is there any way to "restore" a .pst (Outlook Personal Folder) or .ost (Offline Folder file)?

Is there anyway to prevent "Outlook is using an old .ost file" error?

Convert Outlook .ost file to .pst

How to read or reuse and old, saved, Outlook .ost file?

Extract messages from OST/PST files?

I've depleted my resources, so if anyone can point me in the right direction, I'd be very grateful.

  • Have you simply tried to replace the existing empty .ost file with the file in question. Just create a copy with the same name. It wasn't clear if you tried actually tried this or not. At the end of the day the email is only a cache of the contents that was in the mailbox.
    – Ramhound
    Mar 15, 2013 at 17:22
  • I have tried that, it's mentioned above. Outlook just renamed it to .ost.old, created a new .ost and started filling it from the start. I'm not sure what you meant by email is only a cache of the content. Can you clarify that please? Mar 16, 2013 at 20:59
  • However downvoted this question care to explain why? Mar 16, 2013 at 21:00
  • 1
    The .ost file is a local cache of the content on the server. So if the account is no longer valid Outlook will refuse to load the content.
    – Ramhound
    Mar 17, 2013 at 0:17
  • Is this also true for IMAP Outlook 2013 data files? If so, where does Outlook 2013 keep the emails then and how would one go about backuping/transfering them? If it wasn't clear from above, I only have an .ost file being modified and kept up to date, no .pst that I can see anywhere... Mar 17, 2013 at 0:21

1 Answer 1


We had this same problem last week and my experience was almost the same as yours. My final resolution was to export the old outlook data to a pst file and then import it.

  • 1
    I don't understand what you're suggesting.  The OP reinstalled the OS (Windows 7) on his computer, and cannot get Outlook to recognize the .ost files that he backed up.  If he can't get the data into Outlook (and his once-working system has been wiped), how is he supposed to export the "Old Outlook data"? Nov 2, 2015 at 15:05
  • He can get the data into outlook by downloading it again from the exchange server
    – Hennes
    Mar 7, 2016 at 16:28

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