What is Oversized OST File Problem?

Microsoft Outlook 2002 and lower versions limit the size of offline folder (OST) file to 2GB. When the file reaches or exceeds that limit, you will encounter one or more of the following errors:

  • Cannot open or load the OST file at all.
  • Cannot add any new data to the OST file.
  • Cannot synchronize the OST file with the Exchange server.
  • See various error messages during the synchronization process.

This is called oversized OST file problem.

Microsoft Outlook and Exchange have no built-in functions to rescue the oversized OST file. Microsoft only released several service packs so that when the OST file size approaches the 2GB limit, Outlook will show some error messages and stop accepting any new data. This mechanism, to a certain extent, can prevent the OST file from being oversized. But once the limit is reached, you can hardly do anything with the OST file, such as send/receive emails, make appointments, write notes, synchronization, etc., unless you remove bulk of data from the OST file and compact it thereafter to reduce its size to less than 2GB. This is very inconvenient when the data in OST file grow larger and larger.

Since Microsoft Outlook 2003, a new OST file format is introduced, which supports Unicode and doesn’t have the 2GB size limit any more. Therefore, if you are using Microsoft Outlook 2003 and higher versions, and the OST file is created in the new Unicode format, then you don’t need to worry about the oversize problem.


1. When you try to load an oversized OST file, you will see error messages, such as:

Errors have been detected in the file xxxx.ost. Quit all mail-enabled applications, and then use the Inbox Repair Tool.
where ‘xxxx.ost’ is name of the OST file to be loaded.

2. When you try to add new messages or other objects to the OST file, by synchronization or other operations, and during the process, the OST file reaches or goes beyond 2GB, you will find Outlook just stops accepting any new data without complaints, or you will see error messages, such as:

Task ‘Microsoft Exchange Server’ reported error (0x00040820):’Errors in background synchronization. In most cases, further information is available in a synchronization log in the Deleted Items folder.’


Errors in background synchronization. In most cases, more information is available in a synchronization log in the Deleted Items folder.


Can’t copy the item.


As mentioned above, Microsoft doesn’t have a satisfactory way to solve the oversized OST file problem. The best solution is our product DataNumen Exchange Recovery. It can recover the oversized OST file easily and efficiently. To do this, there are two alternative methods:

  1. If you have Outlook 2003 or higher versions installed on your local computer, then you can convert the oversized OST file into a PST file in the new Outlook 2003 unicode format, which doesn’t have the 2GB limit any more. This is the preferred method.
  2. If you only have Outlook 2002 or lower versions installed, then you can split the oversized OST file into several smaller PST files. Each PST file contains a portion of the data in the original OST file, but it is less than 2GB and independent from each other so that you can access it separately with Outlook 2002 or lower versions. This method is a little inconvenient as you need to manage multiple PST files after the split operation. And you still need to face the headachy oversized problem when any PST file reaches 2GB later.