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OST File Size is Bigger Than Mailbox Size Issue

Published By Nilesh Kumar
Debasish Pramanik
Approved By Debasish Pramanik
Published On January 23rd, 2024
Reading Time 7 Minutes Reading
Category Email Tips

Summary: In this article we are going to resolve the problem faced by the Outlook users, OST file size is bigger than mailbox size. Also we will focus on the issues due to OST file size Larger than mailbox size. The user can read this article to know how to tackle with this issue.

Offline Storage Table or OST is the Data file that is the duplicate of a client’s mailbox on the Exchange Server that is privately stored on the PC/framework. This Outlook Data (OST) file holds the whole Outlook mailbox data since the hour of the last synchronization with the server.

The OST file helps the client to work offline if the Exchange Server isn’t accessible.

Even though the OST file is the specific copy of the client’s mailbox, its size doesn’t generally mirror the size of the mailbox. Other than mailbox items, there are numerous different items stored in the file which can take a bigger space. Items stored in the OST file rely on whether you are utilizing an Exchange server or IMAP account. Before discussing why OST File Size is Bigger Than Mailbox Size Let us have a look at mailbox size versus OST file size.

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Mailbox Size Versus OST File Size

You most likely currently observe that the mailbox size and the size of the OST file can be very apart. Be that as it may, in any event, when you would disable all the extra reserving and store nothing in the “This Computer only” folders, the size of the OST file could be very unique in relation to the size of the mailbox. This particularly applies to Exchange accounts.

The OST file can be 20-30% bigger than the mailbox size when utilizing Outlook 2010 or the previous version. This is because of some storage inefficiency of the OST file and the manner in which Exchange figures the mailbox size (Exchange 2013 and later computes it all the more precisely, however).

At the point when you use Outlook 2013 or later versions, the OST file can really be up to 30% smaller than the mailbox size. This is because Outlook 2013 and later compress certain streams of text while storing it in the OST file. You could contrast this with compressing a huge txt-file; it considers compression and along these lines brings about a smaller OST file.

What is in the OST file of an IMAP account?

In Outlook 2013, 2016, 2019, and Office 365, IMAP accounts use OST files also.

In this case, the OST file is also frequently used to reserve the mailbox data that is stored on the IMAP mail server. However, the OST file may also contain some unusual data that isn’t being synchronized with the mail server. These folders are labeled “This Computer Only.”

The accompanying folders don’t match up with the IMAP mail server:

  • Drafts
  • Sent Items
  • Trash/Deleted Items
  • Junk E-mail
  • Calendar
  • Contacts
  • Tasks
  • Notes

Issues Due to OST File Size is Bigger Than Mailbox Size

Because of a larger than average OST file, you may confront the accompanying issues with your Outlook:

  • The outlook may neglect to open or load the OST file
  • An issue in adding data to the existing OST file
  • Outlook Application may interrupt or stop unexpectedly
  • Error messages may show up during the synchronizing procedure
  • Inability to fix synchronization issue.

How to Reduce the Size of the OST File

Reducing the size of the OST file isn’t generally that simple and is not generally required either. On the off chance that you are on a sensibly modern PC and are utilizing Outlook 2007 SP2 or later, your OST file can be as extensive as 10GB you’d even notice the performance.

Regardless, letting it become superfluously enormous isn’t the way either. To decrease the size of the OST file, you can use the accompanying techniques:

  • OST-file Re-creation

After a significant Cleanup, you can decide to let Outlook re-create the OST file as opposed to compacting it or anticipating compression. To do this, rename the OST file to .old when Outlook is closed. After restarting Outlook, your mailbox will be reached in another OST file.

  • Sync Filters

When you have a large number of folders, you can use Synchronization Filters to determine what gets synchronized and stored in the OST file.

  • Sync Slider

In Outlook 2013 and later versions, you can use the Sync Slider to just store later items instead of the whole mailbox. Note that for Outlook 2013 this just applies to your own mailbox and not to any extra shared mailbox you may have connected. For Outlook 2016, 2019, and Office 365, the sync slider applies to all stored mail folders.

  • Disable Caching of shared folders

On the off chance that you don’t require offline access to shared folders and don’t normally get to them, it is possible that you can disable the option to cache shared folders all in all or not store shared mail folders.

  • Disable storing of Public Folders Favorites

Like storing shared folders, you can decide to disable the alternative to cache Public Folder Favorites.

  • Evacuate some Public Folder Favorites

Rather than completely disabling the option, you could also remove some Public Folder Favorites or just select a couple of subfolders to reduce the size of data cached.

  • Gmail duplication

When you use a Gmail account, items may be copied due to the folders you are subscribed to. This additionally applies to Outlook 2010 and past which store the IMAP reserve in a PST file.

If the OST file turns out to be an excessively enormous size, it’s smarter to take a backup and save the file in PST file format. You can convert OST to PST as it very well may be effectively moved to other storage locations and brought into some other Outlook account.

The user can take the help of an expert solution. This tool can without much effort converts a huge size OST file to Outlook PST, inside a couple of clicks. The professional release of the tool can likewise part and minimize huge PST files, accordingly, guaranteeing the ideal execution of Outlook.

Also Read: Reduce Size of Oversized OST file.


When the Outlook OST File Size exceeds the Mailbox Size, reducing the OST file size is unlikely to be required on a consistent basis. Regardless of whether it is required, it is not always extremely simple.

While utilizing any sensibly modern PC and utilizing Outlook-2007-SP2 or any later form, at that point OST file greater than mailbox size as large as 10GB size and you would even now not notice any such performance hit.

Be that as it may, all items considered, permitting it to become greater and greater in size absurdly is additionally not the best approach to work either.