Outlook PST File Too Large – How to Fix the Problem

Introduction

Microsoft Outlook stores emails, attachments and calendar data in files known as PST or OST files. Over time these files can grow very large, especially in environments where users keep years of emails or receive large attachments.

When a PST file becomes too large, Outlook may begin to slow down, crash or display error messages. In some cases the mailbox may stop working entirely.

Fortunately there are several steps that can help resolve the issue and restore Outlook performance.

​​​

Step 1 – Check the Size of Your Outlook Data File

To determine whether the PST file has reached its size limit:

1. Open Outlook

2. Click File

3. Select Account Settings

4. Choose Account Settings again

5. Click Data Files

Here you can see the location and size of the Outlook data file.

Large PST files can sometimes exceed 20GB or even 50GB, depending on Outlook version.

​​​

Step 2 – Archive Old Emails

Archiving older messages is one of the easiest ways to reduce PST file size.

1. Open Outlook

2. Click File

3. Select Tools

4. Choose Clean Up Old Items

You can archive messages older than a specific date to a separate archive file.

This reduces the size of the active mailbox.

​​​

Step 3 – Delete Large Attachments

Attachments are often responsible for most PST file growth.

Steps to identify large emails:

1. In Outlook search bar type:

size:>100MB

​

  2.Review large messages

  3.Delete or save attachments externally

Removing large attachments can significantly reduce mailbox size.

​​​

Step 4 – Compact the PST File

Even after deleting emails, the PST file may remain large until it is compacted.

Steps:

1. Open File → Account Settings → Data Files

2. Select the PST file

3. Click Settings

4. Choose Compact Now

This removes unused space from the data file.

​​​

Step 5 – Consider Using Microsoft 365 Online Mailboxes

Businesses using Microsoft 365 often avoid PST file problems by storing email directly in the cloud mailbox instead of local PST files.

This allows email storage to be managed centrally and reduces the risk of PST corruption.

​​​

Conclusion

Large PST files can cause Outlook performance problems, crashes and data corruption. Archiving old messages, removing large attachments and compacting the PST file can often resolve the issue.

If the PST file becomes corrupted or Outlook continues to crash, professional IT support may be required to repair the mailbox and restore access to email data.

​

​

​

Need Help Fixing This Problem?

Bluewall IT provides professional IT support for businesses across Gosport, Fareham, Portsmouth and Hampshire, helping organisations resolve IT issues quickly and keep their systems running smoothly.