How To Fix Oversized OST File Issues

Error message

The Outlook shows following error messages when OST file becomes larger than defined limit.

  • “An error has been detected in the file xxxx.ost. Then Quit all mail-enabled applications to use Inbox repair tool”.
  • “Cannot copy the items”.
  • “Microsoft Exchange server reported error 0x00040820 error which is for background synchronization”.


The Outlook has size limitations of ANSI for OST file which is 2 GB and for UNICODE .ost file it is 20 GB for Outlook 2007 & for Outlook 2010 & 2013 it is 50 GB. OST files hold entire mailbox data generated while the account was created and keeps on receiving more and more data since then unless any items are deleted. The Oversized OST File issue is due to the over sizing of OST files than defined limit.


The first four solutions are quick fixes. Try them first. If they did not work then try next solutions to reduce OST file size.

1. Rename OST File

If OST file possesses problem in execution of normal operations like opening, sending, receiving, etc. This issue can be resolved by renaming the file, you can just rename OST file & restart Outlook.

2. Run Outlook in Safe Mode

Running Outlook in safe mode can resolve Oversized OST file issue which is faced by the users due to Exchange OST file.

3. Recreate Outlook Profile

As the user is using Outlook with Exchange Server to work with OST file hence, it is required to delete the current profile & create it again. It can fix the error sometimes.

4. Disable All Add-ons

While using Outlook there are some Add-ons which gets enabled & leads to obstruction & disturbances. In such situation disable the add-ons which are installed in Outlook.

5. Delete unwanted items

You can delete unwanted and older emails and other mailbox items to reduce size of OST file. The size will reduce considerably if you delete many old items. You can empty trash folder to delete items permanently.

6. Compact OST file

Compacting OST file will reduce its size at certain extent. Follow below steps to do this.
1. Go to menu > Folder List > Inbox. Then Open properties and select the option “Clear Offline Item”
Go to Tools > E-mail accounts > View or change existing e-mail accounts. Click Next.
2. In the Outlook processes e-mail for these accounts in the following order list, choose the option Microsoft Exchange Server. Click Change.
3. Click More Settings -> Advanced tab -> Offline Folder File Settings then click Compact Now.

7. Auto Archiving of items

You can archive large but important items to reduce size in large extent.
1. Click File tab. Then, click Cleanup Tools.
2. Select Mailbox > Cleanup now. Click the option of Auto Archive to move items to archive items.

8. Use OST to PST converter

You need third party softwares to follow this method. There are various OST to PST converter tools to make your work easier. These tools can repair corrupted OST file, compact OST file. Some tools can split OST file to reduce its size. This is final fix which works almost always.