Know How to Solve Outlook Oversized OST File Problem

The article gives a brief summary of the issues generated due to oversize problem of OST file. In addition, the probable solutions, which enable to solve the issue, have also been discussed.
Offline Storage Table aka OST files are one of the core file formats of MS Outlook. While PST file stores the complete Outlook data, OST file comes into existence when Outlook is used in collaboration with Exchange server. It is rightly referred as the replica of the data that resides on the Exchange server.
Though the structure and functionality of the OST file differs from the other file types, one thing remains the same. Similar to other files, OST files too have a certain pre-defined size limit, that when exceeded, might give rise to a number of issues.

Issues Due To Increased Size Of OST File

The predefined size limit of ANSI OST file is 2 GB, for UNICODE OST file for Outlook 2007 is 20 GB, and for Outlook 2010 and 2013 is 50 GB. When an OST file exceeds this predefined size limits, certain performance issues can be observed in Outlook. In case these issues persist, it can lead to corruption of the OST file. When the number of mails increases in the OST file, the user might delete few of the in order to free up some space. However, technically speaking, this does not free up the file because the deleted emails get stored in the Deleted Items folder and thus reside in the same OST file.
  • Unable to open or load the OST file data.
  • Unable to add any new data to the existing OST file.
  • Failure in synchronizing the OST file with Exchange server.

Difference between Mailbox and OST File Size

An increased OST file size does not always imply that the factor responsible for it is the increased mailbox size. Even though, the mailbox data does constitute the majority part in the OST file, there can also be additional data present in the OST file. In Outlook 2010 and previous versions, the size of OST file can be 20-30% larger than the mailbox size. This can be a result of storage deficiency or issue in calculating the mailbox size from Exchange part.

How To Safeguard Against OST Oversizing

In order to save your OST file from exceeding the defined size limit, you can keep in mind the below mentioned points and can optimize the size of your OST file:
  • Delete all the unwanted items from different folders of your OST file.
  • Hard delete the items you do not require. In addition, empty the Deleted Items Folder in order to get rid of the deleted messages permanently.
  • Compact the size of your OST file.

Methods to Solve Outlook OST File Oversized Problem

The above-mentioned methods will only work for you when you need to save your OST file from getting oversized. In case your OST file has already crossed the size limit, other methods that can be used are:

1. Rename OST File
Sometimes, the OST file may pose issues in performing normal operations in Outlook like sending, receiving, opening, etc. This issue can be resolved by renaming your OST file.  Just rename your OST file, restart Outlook and you are good to go.

2. Run Outlook In Safe Mode
Running Outlook in safe mode may resolve the common issues that are experienced by you due to OST file.

3. Recreate Your Outlook Profile
Since you are using Outlook with Exchange server in order to work with OST files, therefore you will need to delete the current Outlook profile and e-create it.

4. Disable The Add-Ons
While working with Outlook, there might be some add-ons that might be hampering and disrupting its normal functioning. In such cases, it is better to disable the add-ons installed in Outlook.

5. Archive OST File Data
Archiving OST file data in a new file may help in emptying the space occupied on the server. This ensures that the data stored on the server does not cross the pre-defined size limit.  The archive file can be saved at any desirable location on the user machine.

6. Disable Anti-Virus
In case none of the above issues works out for you and your Outlook performance is still the same, then try disabling the anti-virus and firewall present in your system. Disable them on a temporary basis and see of this makes any difference to Outlook performance.

We believe that by now you might have been well acquainted with the reasons that lead to oversize issue in Outlook OST files. You can save Outlook from experiencing oversize OST file issues by following the safeguard methods provided. However, in case the OST file has already gone overboard, then try out the remedial measures discussed in the above section.

Comments (3)

Although I believe the article is a good start in identifying issues that cause OST degradation (and I'll admit I hadn't considered this myself, having run into this behavior before), I didn't find it went far enough in helping the layman understand where to find/execute the correcting actions. Something as simple as a breadcrumb trail to navigation (i.e. goto Outlook > Options > etc. > etc.) would have been quite useful for those, such as the older ones that often work in my office, who aren't as familiar with the backend of the application. I think it might be important to remember that not all members of the community are experts, or maybe just not an expert in the specific field discussed, and those small details can make a big difference when trying to resolve the issue in terms of man hours that might be needed in searching and reading other articles to piece it together. Good article, no doubt, just maybe needs a little more in the how-to department. Keep 'em coming!
Our service desk has a batch file to run on users that are having OST or Outlook issues. It deletes the following files and folders from "%userprofile%\AppData\Local\Microsoft\Outlook": *.OST, *.TMP, *.OBI, *.INF, *.XML, "Offline Address Books", and "RoamCache". Do not use on a user unless they are in the office or on a fast connection since OST rebuilds can be a slow.

TASKKILL /F /IM "outlook.exe"
TASKKILL /F /IM "communicator.exe"
TASKKILL /F /IM "UcMapi.exe"
Timeout /T 5 /NOBREAK

attrib -H "%userprofile%\Local Settings\Application Data\Microsoft\Outlook\*.*"

del "%userprofile%\Local Settings\Application Data\Microsoft\Outlook\*.ost" /q /f
del "%userprofile%\AppData\Local\Microsoft\Outlook\*.ost" /q /f

del "%userprofile%\Local Settings\Application Data\Microsoft\Outlook\*.tmp" /q /f
del "%userprofile%\AppData\Local\Microsoft\Outlook\*.tmp" /q /f

del "%userprofile%\Local Settings\Application Data\Microsoft\Outlook\*.obi" /q /f
del "%userprofile%\AppData\Local\Microsoft\Outlook\*.obi" /q /f

del "%userprofile%\Local Settings\Application Data\Microsoft\Outlook\*.inf" /q /f
del "%userprofile%\AppData\Local\Microsoft\Outlook\*.inf" /q /f

del "%userprofile%\Local Settings\Application Data\Microsoft\Outlook\*.xml" /q /f
del "%userprofile%\AppData\Local\Microsoft\Outlook\*.xml" /q /f

rd /s /q "%userprofile%\Local Settings\Application Data\Microsoft\Outlook\Offline Address Books"
rd /s /q "%userprofile%\AppData\Local\Microsoft\Outlook\Offline Address Books"

rd /s /q "%userprofile%\Local Settings\Application Data\Microsoft\Outlook\RoamCache"
rd /s /q "%userprofile%\AppData\Local\Microsoft\Outlook\RoamCache"

Marshal HubsEmail Consultant

You can resolve oversize OST file issue by using following methods:

  •    Delete the unwanted mailbox items
  •    Empty the Deleted Items folder
  •    Compact the ost file

Refer this article for more information:

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