Tips to Manage Gradually Increasing Size of OST Files in Outlook

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Email system is the most inexpensive, fastest, and reliable means of communication across the globe, and is preferred mostly in corporate to exchange crucial data inside and outside their network domain. Unlike individuals, organizations prefer having their own email server, usually Microsoft Exchange Server in collaboration with MS Outlook, for reliable and secure emailing environment.


When Outlook synchronizes with Exchange Server, entire mailbox contents, such as Emails, Contacts, Calendar, Notes, Tasks, and more are stored on the server that makes them accessible globally.

Besides, a copy of the mailbox is located on the client computer and is termed the Outlook Data File that is of two distinct types (i.e. Outlook.pst and Outlook.ost) and both function differently. Well, OST files are strongly associated with Outlook in collaboration with Exchange Server. Go through the below section for detailed overview of the OST files.

The OST File

The term OST stands for Offline Storage Table, i.e. the Offline Outlook Data File, which is specific to the Exchange Server in collaboration with MS Outlook (as mentioned above). The OST file provides offline access to the mailbox and is stored locally, i.e. on the client’s computer.

Well, providing offline access means when the Internet connectivity is no longer available. Meanwhile, every single modification done in the local mailbox (i.e. in the Outlook.ost file) will also be done in the mailbox on the Exchange Server whenever the Internet connection is reestablished. In other words, synchronization leads to balanced mailboxes contents.

OST File in Different Outlook Versions

Since the OST file is uniquely associated with MS Outlook in collaboration with the Exchange Server, its storage limit varies when associated with different versions of Outlook. More precisely, there is a limit of the size of the OST file associated with different versions of MS Outlook.

When it comes to Outlook 2013 and its predecessor Outlook 2010, the maximum size of both PST and OST files cannot be greater than 50 GB. On the other hand, Outlook 2007 and Outlook 2003 limit the size of their data files (both PST and OST) to 20 GB. The reason of such large sizes is the Unicode format of the data files, while earlier versions of Outlook (earlier than 2003) used ANSII format with the size limit of 2 GB only.

Issues with OST file Size

As mentioned above, the OST file associated with each version of MS Outlook in collaboration with the Exchange Server has a certain size limit. An OST file with its size close to the storage limit may lead to corruption at any time. For instance, professionals in corporate exchange around hundreds (or even more) of emails every day, which keeps increasing the size of the mailbox and its corresponding OST file located on the client computer.

This not only eats up large drive space, but it may also cause corruption and some like issues in the OST file. In other words, the gradually increasing size of the mailbox is eventually leading it to a state of failure.

Managing Size of the OST File

Based on the number of emails (plus attachments) created and received every day, the overall size of the mailbox varies. Since there is a storage limit for each mailbox, the gradually increasing mailbox size is leading to corruption or failure at the end of the day. Hence, there is a need for managing the size of the mailbox (i.e. the OST file) to avoid the loss of crucial data.

Well, deleting the older and all unwanted emails may not favor the OST file as expected, but compacting the OST file will surely do. See how to compact the OST file in Outlook in Outlook 2010:

  1. Launch Outlook and delete all unwanted items in the mailbox.
  2. Right-click the Deleted Items folder in mailbox, and then click Empty Folder to delete them permanently.
  3. Now, click File tab and select Account Settings in the context menu.
  4. Click Account Settings, and then click Data Files tab.
  5. Select the target data file, and then click Settings.
  6. Click Compact Now button.

Compacting the mailbox process results in reduced size of the OST file. In case Outlook crashes or fails to open due to unknown issues, there might be issues with the OST file or the Outlook client.

Repairing Corrupt Outlook Mailbox

Corruption in the Outlook Data File (i.e. PST or OST) sometimes causes Outlook inaccessibility. In such scenarios, the mailbox needs to be repaired to regain access to the emails and other stuff.

Well, Outlook includes Inbox Repair Tool (Scanpst.exe) that can repair both the Outlook Data Files (i.e. OST and PST) in an efficient manner. Since this is an inbuilt tool, it gets installed automatically with Outlook client and its default location on the client computer (in Outlook 2010) is:

C:/Program Files/Microsoft Office/OFFICE12

To repair the inaccessible Outlook client or the corrupt the OST file, visit this location and launch the Scanpst.exe tool. Meanwhile, keep Outlook closed. See how to repair corrupt OST file:

  1. With the Scanpst.exe tool running, click Browse and locate the corrupt OST file.
  2. To enable scan logs, click Options and select required options, and then click Start.
  3. The tool prompts to start the repair process if it finds the errors. Click Repair to fix the errors.
  4. Once OST repairing is finished, launch Outlook and see if it worked.

Inbox Repair Tool (Scanpst.exe) is reliable and efficient to fix such issues. However, in case it fails to fix the issues encountered, it is recommended to use commercial OST repair software to avoid the loss of crucial data. Using OST repair tool, locate and scan the corrupt OST file stored at the default location (as mentioned earlier), and then start repairing it to recover mailbox data intact.

Important: Mailbox corrupt might not be a big issue for individuals, but it undoubtedly might cause loss of crucial information for professionals who send/receive hundreds of emails every day. Therefore, taking regular backup is a wise and productive decision in the long run.


Vishal Gaikar

Article by Vishal

Meet Vishal Gaikar, the tech wizard hailing from Pune, India, who's on a mission to decode the digital universe one blog post at a time. When he's not tinkering with gadgets or diving deep into the digital realm, you can find him concocting the perfect cup of chai or plotting his next adventure. Follow his tech escapades on Twitter and buckle up for a wild ride through the world of innovation and geekery!

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