Why OST File is Larger than the Mailbox?

Offline Storage Table or OST is the Outlook Data file that is the exact copy of a user’s mailbox on the Exchange Server that is locally stored on the PC/system. This Outlook Data (.ost) file holds the entire Outlook mailbox information since the time of the last synchronization with the server.

The OST file helps the user to work offline in case the Exchange Server is not available.

Though the OST file is the exact replica of the user’s mailbox, its size does not really reflect the size of the mailbox. Besides mailbox items, there are many other items stored in the file which can take larger space. Items stored in the OST file depend upon whether you are using an Exchange server or an IMAP account.

Note: If you’re using Outlook 2013 or a higher version, the size of your OST file can also be lesser than the mailbox. This is because Outlook version 2013 and above compress some specific text streams while storing them in the Outlook Data (.ost) file.

Why OST File is Larger than Mailbox?

Following are some common items that are saved in the OST File of the Exchange server account, which can result in an OST file size larger than the mailbox:

• Mailbox Items such as email, calendar, tasks, journal folders, contacts, notes, etc.
• Hidden Items and other overheads are found in the PST file.
• Public folders that are added by the user as favorites.
• Shared folders of additional mailboxes, which have been connected to the Exchange server account.
• In Outlook 2016, when you connect to Exchange Server, there are two connectivity modes – Cached Exchange Mode and Online Mode. With the Cached Exchange Mode (a default setting for users), Outlook 2016 doesn’t depend on constant network connectivity for accessing the users’ information. However, Outlook needs to directly contact the Exchange server to perform certain operations, even when it is configured in Cached Exchange Mode. These operations are working with shared folders that were not made available offline, setting/changing/canceling Out-of-Office message, accessing public folders which were not made available offline, editing rules, and recovering rights to a rights-protected message, and recovering Policy Tips, Mail Tips & Free/Busy information. If you’re using Outlook 2016 with Cached Exchange Mode, it is to be noted that users’ OST files, stored locally, might be 50-80 percent larger than the mailbox size. With the Cached Exchange Mode, the format that Outlook 2016 uses to save data locally is less space-efficient as compared to the server data file format.

Problems with Outlook due to Oversized OST

Due to an oversized OST file, you may face the following issues with your Outlook:

• The outlook may fail to open or load the OST file document
• The problem in adding data to existing OST file
• Outlook Application may pause or stop abruptly
• Error messages may appear during the synchronizing process
• Failure to fix synchronization problem

How to Reduce OST File Size?

You can use Outlook’s built-in Mailbox Cleanup tool to view the size of the mailbox and different folders. This built-in utility can help search for large or older messages that you might want to delete or export to a PST file.

Often, the major reason for an oversized mailbox is a big‘Deleted Items’ folder. The Mailbox Cleanup tool offers an option to view the Deleted Items folder size and to empty the folder.

Apart from emptying the Deleted Items from your mailbox, there are various ways to reduce the size of the OST file. You can try the following methods:

Method 1: Compact the OST File to Reduce its Size

When you delete items from Outlook, it doesn’t reduce the size of the OST file. The space, from where the items were deleted, becomes a ‘white space’, which stores nothing.

By compacting the OST file, you can optimize the file storage process, with no white spaces. It is to be noted that there is no data loss if you compact the OST file. You can compact the file easily from the option given in Outlook settings.

Method 2: Recreation of OST File

After doing a major mailbox cleanup, you have a choice to let the Outlook application recreate the Outlook Data (.ost) file without compacting it or you can wait for the automatic compression. You can also recreate the OST file manually.

To do this, read How to Recreate OST file in Microsoft Outlook 2007/ 2010/ 2013/ 2016.

Method 3: Apply Synchronization Filters

In several circumstances, where there is the involvement of shared folders, it is usual to find the recent items in those folders and not others. If you have some very large folders, there is an option to set Synchronization Filters. This determines what items get synchronized and saved in the OST file as it caches only the items received recently based on the filter set such as items received last week/month or any other date specified.

Synchronization filters can help in considerably reduce the cached items and the OST file size.

Method 4: Apply Sync Slider to Reduce OST Size

In Outlook 2013 and higher versions, there is a feature ‘Sync Slider Setting’ that only caches more recent emails such as of 1 month, 3 months, 6 months, or 1 year, rather than the complete mailbox.

If you’re using Outlook 2013 or above version, you can use this Outlook feature to cache the latest email items and reduce the Outlook Data (.ost) file size.

You can find the Sync Slider in your Outlook by following: File ->Account Settings->Account Settings…-> click on E-mail-> double-clicking on your Exchange email account.

Read More, What is the Outlook Cached Mode Sync Slider?

Note: In Outlook 2013, the Sync Slider can only be applied to your own mailbox (i.e., the main mailbox that is linked to the Exchange account). It doesn’t apply to caching any additional shared folders. However, in Outlook 2016, 2019, and Office 365, the sync slider can also be applied to all the mail folders.

Method 5: Disable the Caching of Shared Folders

If you don’t want any offline access to your shared folders and don’t access these folders regularly, then you can choose to disable the option to cache shared folders. Besides, you can also choose the option to not cache the shared mail folders. This can help in reducing the size of the OST file.

Method 6: Reduce OST File Size by Disabling the Caching of Public Folders Favorites

If the size of OST is larger than the mailbox, you can also try to disable the caching of public folder favorites. For this, you can choose the option to disable the caching of public folder favorites, just like the shared folders.

Method 7: Try to Remove Some Public Folders Favorites

If you don’t want to entirely disable the caching of public folder favorites, you can choose the option to remove only some public folder favorites. Besides, you can select only a few subfolders rather than reducing the data volume that is being cached.

Method 8: Apply Archive Policies to Reduce Mailbox Size

To improve the performance and reduce the size of the mailbox, you can automatically move emails from the main mailbox to the archive mailbox by using the Exchange Archive Policies.

When you apply the Archive Policies to a user’s mailbox, messages from the primary mailbox automatically move to the archive mailbox after a set time period. You can apply Archive Policies by creating retention tags that use the ‘Move to Archive’ retention action.

To Conclude

Microsoft Outlook mailbox increases as you receive emails and create items, thus making the OST file larger than the mailbox. However, if you look at the OST file size, you will find that it does not actually represent the mailbox size but it also stores many other items. This may result in the OST file being larger than the mailbox.

A larger or oversized OST can create issues in Outlook performance or is prone to corruption and other problems. You can reduce the size of the OST file by following the methods given in the post.

In case, the OST file becomes too large, it’s better to take a backup and save the file in PST format. You can convert OST to PST as it can be easily moved to other storage locations and imported into any other Outlook account.

To convert a large-sized OST file to PST, you need a specialized OST to PST converter tool such as Stellar Converter for OST.

This DIY software can easily convert a large-size OST file to PST, within a few clicks. The technician edition of the software can also split and compact large PST files, thus ensuring the optimal performance of Outlook.

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About The Author

Charanjeet Kaur

Charanjeet is a Technical Content Writer at Stellar®who specializes in writing about databases, e-mail recovery, and e-mail migration solutions. She loves researching and developing content that helps database administrators, organizations and novices to fix multiple problems related to MS SQL and MySQL databases and Microsoft Exchange.