Learn Simple Tricks to Fix Oversized OST File Issue

admin | July 4th, 2017 | Outlook

Microsoft Outlook being one of the widely used email applications in organizations saves its database in two file formats i.e. PST and OST. However, they are part of the same application but their working is different from each other. OST file comes into play when MS Outlook is configured with Exchange Server by enabling MAPI protocol. OST files are a replica of the data residing on the Exchange Server. The OST file acts as a repository, which stores entire mailbox data which includes emails, calendars, contacts, etc. These files are useful for the users who desire to work in offline mode either due to Exchange Server configuration issues or when the Internet traffic is at its peak. But there is file size limitation in OST files which creates many issues when its size limit exceeds. In this article, we will discuss Oversized OST File issue and a reliable way to solve it.

Common Reasons Behind Increased OST File Size

Outlook has predefined the file size limit of the ANSI OST file i.e. 2 GB, UNICODE OST file has 20 GB size limit for Outlook 2007 and 50 GB limit for Outlook 2010 and 2013. Whenever the .ost file surpasses the predefined file size limit then there occur some performance issues in Microsoft Outlook. If these issues continue to occur, then it can lead to OST file corruption. Observing this issue, some users try to delete the emails from their OST emails but it doesn’t result in the reduction of OST file. Those emails will get saved into the Deleted Items folder and will stay in the same .ost file. So, the Oversized OST File Size remains same.

Difference between Mailbox and OST File Size

The increased file size does not always mean that the mailbox size is increased. Although the mailbox data does constitute the large part of the OST file, there can be chances that .ost file is containing some additional data. In Microsoft Outlook 2010 and below versions, the size of OST file can be 20-30% larger than the size of the mailbox. It can be the result of storage deficiency or issue in calculating the mailbox size.

Ways to Maintain Oversized OST File Size Issue

If the OST file has not yet exceeded the file size limitation and users want to save it from getting crossed the limit, then they must keep the below-mentioned points in mind. These points help them to optimize the file size of the Outlook Offline Storage Table.

(1) Hard delete those items which are not required.
(2) Keep the Deleted Items folder empty in order to get permanently rid of the deleted messages.
(3) Delete all the items which are not required from Outlook .ost file.
(4) Compress the size of OST file.

Approach to Resolve Oversize Outlook OST File Issue

The above-mentioned methods only work if a user wants to save his/her from getting oversized. If the OST file has already crossed the file size limit, then there are two ways to solve it.

Remedial Methods

Rename OST File – Most of the times, OST files may pose problems while executing normal operations such as opening, sending, receiving, etc. It can be resolved by renaming OST file.

Recreate Outlook Profile – Since the user uses Outlook with Exchange Server to work with OST files. So, it is required to delete current Outlook profile and create it again.

Run Outlook in Safe Mode – Another solution to solve this problem is that users can also try by running MS Outlook in safe mode as it may also help.

Archive OST Data – Archiving Offline Storage Table’s data in a new file may help in clearing the undesired space which is occupied on the server. This method safeguards the data which is stored on the server from crossing limit of the file size.

Expert Solution

All the above-discussed methods might not work in some cases as they are just hit and trial methods. If all these methods don’t work then users must go for a third party solution. OST File Recovery Software is one such solution as it provides users with an option to repair and export OST files into PST file format. While exporting Offline Storage Table into Personal Storage Table, it offers to split PST file into equal small parts. It is a reliable solution which can solve Oversized OST File problem without any data loss.


There are many users who face issues due to their Oversized OST file due to which they have to face various performance issues while using Microsoft Outlook. In this article, some common reasons behind the problem and manual methods to resolve the issue are explained. Along with it, an expert third party application is specified to have an efficient and reliable solution to the problem.