OST File is Larger than Mailbox – Why and How-to solve

Updated On - December 14, 2023 by Armaan | Reading Time: 6 minutes

In today’s digital world, emails are an essential means of communication, especially in business settings. As such, it’s important to understand how email storage works to ensure that everything runs smoothly. When it comes to email storage, one concept that can be a bit confusing is OST files. These files are larger than the mailbox they represent, and many people wonder why this is the case. In this blog, we will explore why OST file is larger than mailbox and provide a thorough explanation.

What is an OST File?

Before we dive into the reasons why OST files are larger than the mailbox, let’s first define what an OST file is.

OST stands for Offline Storage Table, and it is a file format used to store a local copy of an Exchange mailbox. When you configure Outlook to work with an Exchange account, the OST file is automatically created and used to store a copy of your mailbox data on your local computer.

Why is OST File Larger than the Mailbox?

Now that we have defined what an OST file is, let’s explore the reasons why it can be larger than the mailbox it represents. Here are some of the most common reasons:

Cached Exchange Mode

When you configure Outlook to use Cached Exchange Mode, it will create an OST file that contains a copy of all your mailbox data, including emails, contacts, calendar items, tasks, and notes. This local copy is used to enable offline access to your mailbox, which means you can still view and work with your mailbox data even if you are not connected to the Exchange server.

The cached data is stored in the OST file and is synchronized with the Exchange server whenever you are online. However, this synchronization process can result in changes being made to the OST file, which can cause it to grow in size over time.

Deleted Items

Another reason why OST files can be larger than the mailbox is that they retain deleted items. When you delete an email or other item from your mailbox, it is not immediately removed from the OST file. Instead, it is marked as deleted and remains in the file until it is purged.

This is done to enable recovery of deleted items, but it can also contribute to the OST file’s size. Over time, as more items are deleted, the OST file can grow larger and larger, even if the mailbox itself is not increasing in size.

Mailbox Size Limitations

Exchange servers often have mailbox size limitations that restrict the amount of data that can be stored in a single mailbox. When this limit is reached, the mailbox will no longer accept new data, and users will be unable to send or receive emails.

To prevent this from happening, Outlook will automatically create an OST file that contains a copy of the mailbox data. This copy is used to enable offline access to the mailbox, but it can also cause the OST file to be larger than the actual mailbox.

Attachments

Email attachments can be a significant contributor to the size of an OST file. When you receive an email with an attachment, the attachment is stored in the OST file along with the email itself. This can cause the OST file to grow in size quickly, especially if you receive a lot of emails with large attachments.

Synchronization Issues

Synchronization issues between the OST and the Exchange can also cause the OST file to be larger than the mailbox. When these issues occur, Outlook may attempt to resynchronize the OST file, which can result in duplicate or conflicting items being created. These duplicate items can cause the OST file to grow in size over time.

How to Manage OST File Size

Now that we understand why OST files can be larger than the mailbox, let’s explore some strategies for managing OST file size:

Compact the OST File

One way to reduce the size of an OST file is to compact it. This process removes any deleted items that are still stored in the file and compresses the remaining data. To compact an OST file, open Outlook, go to the File menu, select Account Settings, and then click on Account Settings again. Next, select the Data Files tab and choose the OST file you want to compact. Click on the Settings button and then click on Compact Now. This process may take some time to complete, especially if the OST file is large.

Delete Unnecessary Items

To reduce the size of an OST file, you can also delete any unnecessary items that are stored in the file. This includes emails, calendar items, contacts, tasks, and notes that are no longer needed. To do this, simply select the items you want to delete and press the Delete key. Note that deleting items in the OST file will not delete them from the Exchange server.

Archive Old Items

Another way to reduce the size of an OST file is to archive old items. Archiving moves items that are no longer needed to a separate PST file, which can be stored seperatly. This process frees up space in the OST file and makes it more manageable. To archive items in Outlook, go to the File menu, select Options, and then click on Advanced. Next, click on AutoArchive Settings and set the desired options.

Disable Cached Exchange Mode

If you don’t need offline access to your mailbox, you can disable Cached Exchange Mode in Outlook. This will prevent Outlook from creating an OST file and reduce the amount of storage space needed on your computer. To disable Cached Exchange Mode, go to the File menu, select Account Settings, and then click on Account Settings again. Next, select the Exchange account you want to modify and click on Change. Deselect the Use Cached Exchange Mode option and click on Next.

Increase Mailbox Size Limitations

If you are reaching the mailbox size limitations, you can ask your IT department to increase the limit. This will allow you to store more data in your mailbox without having to rely on the OST file. Note that increasing mailbox size limitations may require additional storage space on the Exchange server.

Author’s Suggestion

If your OST file becomes too large take a backup of your OST file in PST format. This will reduce the risk of data loss. The PST format is a widely supported file format that can be easily moved to other storage locations. Hence, you can import it into any other Outlook account.

This ensures that your important data is always safe and accessible, even in the event of a system failure or other unforeseen circumstances.

You can easily convert your oversized OST file to PST format using automated tools like Regain OST Converter. The tool allows converting OST files to PST format with just a few clicks and without any data loss risk.

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Conclusion

OST files are an essential component of using Outlook with an Exchange account. They enable offline access to your mailbox and provide a local copy of your data that can be synchronized with the Exchange server. However, due to various factors, OST files can be larger than the mailbox they represent. By implementing the strategies outlined above, you can manage the size of your OST file and enhance your Outlook experience.

Frequently Asked Questions

Ques1. Can I delete items in the OST file without affecting the Exchange server?
Ans. Deleting items in the OST file does not remove them from the Exchange server. It only removes them from the local copy on the user’s computer.

Ques2. What is the recommended approach for managing an oversized OST file?
Ans. The author suggests backing up the OST file in PST format using tools like Regain OST Converter. This mitigates the risk of data loss and ensures accessibility in the face of system failures or unforeseen events.

Ques3. What role do email attachments play in OST file size?
Ans. Email attachments are stored within the OST file along with the associated emails. Receiving emails with large attachments can significantly contribute to the growth of the OST file.

Ques4. Can synchronization issues impact the size of the OST file?
Ans. Yes, synchronization issues between the OST file and the Exchange server can lead to duplicate or conflicting items, causing the OST file to expand over time.