Managing a large or oversized Outlook mailbox is a common problem for many users. Although deleting old emails is an obvious solution to fix this problem but many users don’t want to delete their old emails. As an OST file has a size limit so it is important to keep your OST file below the prescribed size limit. Because if your OST file becomes too large, then it may affect your day-to-day work and productivity. In this guide, we will explain the common performance problems and their solutions to optimize Outlook for large mailboxes.

Common Performance Problems Caused by Large Mailboxes

When an Outlook mailbox becomes too large, it can cause several performance issues resulting in problems such as:

  • Freezing, slow loading, and crashes
  • Delayed sending/receiving and sync problems
  • Search not working or returning incomplete results
  • Delayed email Synchronization
  • Risk of corrupt Outlook data file (OST)
  • Slower Exchange server migration

Methods for Optimizing Outlook for Large Mailboxes

To optimize Outlook for large mailboxes, you have to check the size of your PST file by following the steps below:

  • Launch Outlook and go to File.
  • Select Account Settings > Account Settings
  • Choose your Outlook account, click on Data Files, and then click on Open File Location.
  • Now, you can check the size of your OST file. You can see the Size column.

If your OST file is too large the follow the methods mentioned below to reduce the size of your OST file:

  1. Use Mailbox Cleanup Tool

Microsoft Outlook provides a built-in tool called Mailbox Cleanup to reduce the OST file size. You can use this tool to find, manage, and remove unnecessary emails. Follow the steps below to use this tool:

  • Open Outlook and click on File.
  • Click on Tool and select Mailbox Cleanup.
  • On the Mailbox Cleanup window, you can do the following:
  • View Mailbox Size.
  • Find items older than
  • Find large items
  • Empty Deleted Items Folder
  • Archive Items
  1. Adjust the Sync Slider

Outlook provides a sync option in Outlook, which allows you to specify how much mailbox data from the server is downloaded and stored in your OST file. If the sync slider is set to “All”, then Outlook will download all mailbox data from the server, which will cause an increase in the OST file size. To reduce the size of the OST file, you can adjust the slider and set the duration to 6 or 12 months. Follow the given steps to adjust the sync slider settings:

  • Launch Outlook and go to File > Info > Account Settings > Account Settings.
  • Select your email profile and click on Change.
  • Under Offline Settings, you will see the Mail to keep the offline slider.
  • If the slider is set to All, move it to 6 or 12 months. You can also adjust it to any other duration according to your requirements.
  • After adjusting the slider, click on Next and then click on OK.
  • Restart Outlook to apply the changes.
  1. Archive Old Emails

You can use the AutoArchive option to archive old emails to an archive PST folder, which helps you reduce the size of your mailbox. Follow the given steps to archive mails:

  • Open Outlook.
  • Go to the Files tab.
  • Click on Options and select Advanced.
  • Check the box “Run AutoArchive every n days” and set the frequency of archiving.
  • Click

Note: If the AutoArchive option is not visible, it may have been disabled by your IT administrator.

  1. Compact the OST File

Even after deleting your emails and other mailbox items, the OST file size does not reduce immediately. Outlook marks this deleted space as white space. You can compact the OST file to remove the white space and reduce the overall file size. Follow the instructions below to compact the OST file:

  • Launch Outlook and click on
  • Go to Account Settings > Account Settings.
  • Click on Data Files, select your Outlook account, and click on
  • Go to Advanced and click on Outlook Data File Settings.
  • Now, select your OST file, click on Compact now, and then click

You can reduce the size of the OST and improve Outlook’s performance file by following the steps mentioned above.

  1. Disable the Download Shared Folder Option

If you have several shared folders which is linked to your account, Outlook automatically downloads their data into OST file, thus increasing its size. You can prevent downloading the shared folders data into your OST by turning off the option “Download Shared Folder”. This helps keep your OST file size under control and improve Outlook’s performance. Follow the given steps to disable this option:

  • Open Outlook. Go to File > Account Settings > Account Settings.
  • Choose Data files in the Account Settings window and select the Outlook Data File (.ost).
  • Select Settings and then choose the Advanced tab.
  • Uncheck the Download shared folder under Use Cached Exchange Mode, click on Apply, and then click OK.

Best Practices for Improving Outlook’s Performance

  • Keep your mailbox size small by archiving old emails.
  • Empty Deleted Items and Junk folders regularly.
  • Disable unnecessary add-ins.
  • Keep Outlook and Windows updated.
  • Reduce OST size by compacting the data file.
  • Create new Outlook profiles if performance slows down.
  • Avoid using very large OST files.
  • Ensure a stable internet connection for Outlook syncing.
  • Use Online Archive mailbox if available.

Conclusion

If your OST file becomes too large, then it may affect your Outlook’s performance. In this article, we have mentioned the way to optimizing Outlook’s large mailbox and improve its performance. You can follow the methods like using the mailbox cleanup tool, turning off the download shared folder option, etc., to reduce the OST file size.

However, if your OST file becomes inaccessible or corrupted due to its large size, then you can use an advanced OST to PST converter tool, such as Stellar Converter for OST, to extract data from such a file. You can convert inaccessible or orphaned OST files of any size into PST and other formats. Also, you can use its auto-split option to split the converted PST file into multiple smaller files.