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How to Reduce OST File Size in Outlook – Here The Complete Guide

Published By Deepa Pandey
Ashwani Tiwari
Approved By Ashwani Tiwari
Published On November 28th, 2023
Reading Time 5 Minutes Reading
Category MS Outlook

Seeking a solution to reduce OST file size in Outlook? Then, this article describes the best practices for organizing large-size OST mailboxes, avoiding OST file corruption and other collateral problems in Outlook 2019, 2016, 2013, 2010, and below.

Generally, Microsoft Outlook creates a local copy of mailbox data which is also known as the OST file, which is created if the user configured its Outlook account with IMAP or Exchange servers.

It is especially helpful with a slow internet connection or no internet connection. Because if your internet connection is interrupted due to any reason you can still access your mailbox data also you can read and compose messages, create appointments, and add contacts without connecting to the server.

Quick Solution

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What are the Consequences of Large OST Files?

Commonly large OST mailbox data tend to slow down Outlook programs, file corruption, break the search engine indexing, and many other issues. Also, create some errors, the major ones are listed below:-

  • Stuck with the message requesting data from the Microsoft Exchange server
  • Retrieving data from exchange server
  • Outlook email client is not responding
  • Cannot add additional data to existing Exchange .ost file.
  • Delay in the opening of the OST file document.

Note:- Outlook pst folder not showing emails

How to Reduce OST File Size in Outlook?

If you are facing the issue of an oversized PST file, then in this section we are going to discuss some effective techniques to decrease the size of the file which will help you to improve the performance and workflow of the Outlook program.

Reduce the Number of Cached Emails

When one can create the new Outlook profile then it is set by default to keep all mail offline. Since all mail is stored on your server, then there is no need to save all the email messages offline. Because the unnecessary emails will fill the mailbox.

To reduce OST file size in Outlook you can change the number of email messages that are cached from all to 4 or 1 month. Follow the steps below to change the cached emails.

Note: Microsoft is recommended to keep the OST file below 10 GB. And having more than 5000 items in one folder can badly affect the performance of the Outlook program.

  • First open Outlook program.
  • Click on the File option then Info.
  • Now, select the Account Settings option >> Account Settings and select the email address.
  • Click on the change.
  • Now, decrease the Use Cached Mode to 5 or 1 month.

Compact the OST File

To compact, the ost file in Outlook follow the listed below steps

  • First Open the Outlook program and click on the File tab on the Ribbon.
  • Now, choose the Info option.
  • Then go to the Account Settings option. >> Account Settings.
  • Now, choose the More Settings option and then Advanced.
  • After that select Outlook Data Files Settings.
  • At last select Compact Now and thenĀ OK to finish the process.

Switch Outlook to Online Mode

As we said above when a new Outlook profile is created it is set by default to keep all mail offline. So you can use Outlook in online mode which makes it instantly keep the connection to the server and view the data there. If you use Outlook online more than it does not store items locally on the computer, as a result, the OST file does not become oversized and does not get corrupted.

Here are the steps to enable Inline mode in Outlook:

  • First, open the Outlook email client then, go to the File option.
  • After that, select the Info option and select Account Settings >> Account Settings.
  • Now, choose the email address and click on the Change option.
  • Uncheck the Use Cached Exchange Mode >> Next and restart the Outlook program.

Delete Duplicate Items Form OST File

Duplicate items are the major factor to increase the size of the Outlook OST file. So to delete duplicate emails, contacts, attachments, calendar entries, or many more you can try the default Outlook CleanUp utility. But this utility is not effective at all because there is no guarantee that it will delete the duplicate files from the Outlook OST file and reduce the OST file size of Outlook.

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Hence, in this troublesome situation try an effective and verified toolkit that is specially developed for deleting duplicate items from Outlook OST files. Also, it is a single toolkit that is capable of removing duplicate emails from Outlook too. To test the tool you can download the trial version for free and delete duplicate items from Outlook.


Outlook users suffer from many issues including data corruption, performance degradation, and many more due to oversized OST files. Hence, in the above, we have discussed some DIY techniques to reduce OST file size in Outlook. In addition, we also discussed automated solutions to deal with oversized Outlook OST files in a smart way.