Microsoft gives five ways to shrink a mailbox and an Outlook data file. Mailbox Cleanup finds large and old items and empties Deleted Items. Searching messagesize:>5 mb finds large messages. Outlook compacts a PST file on its own when it has more than 20 percent white space. Compact Now forces the step after you delete items.
You delete mail and the PST file remains large. Outlook opens slowly and the mailbox nears its limit. Microsoft explains the cleanup tools and the compact step.
Steps follow the Microsoft Support page on reducing the size of your mailbox and Outlook data files. Microsoft names Outlook for Microsoft 365, Outlook 2024 and Outlook 2021. Microsoft gives no numeric file size limit there and no steps for Archive or AutoArchive.
Which Data Files does Outlook Use?
POP accounts store all account data in a PST file. Microsoft 365, Exchange, IMAP and Outlook.com accounts store a copy of the mailbox in an OST file. Deleting items does not always shrink a PST or OST file.
What are the Five Ways to Reduce Size?
- Mailbox Cleanup. Select File, Tools and Mailbox Cleanup. Options include View Mailbox Size, Find items older than, Find items larger than, Empty Deleted Items and Empty Conflicts. Microsoft links a large Deleted Items folder to a large mailbox.
- Conversation Clean Up. This feature removes redundant messages from long threads. Microsoft links a separate page for the steps.
- Large message search. Type
messagesize:>5 mbin the search box. Delete the results or move them to another PST file. - Automatic compaction. Outlook compacts a PST or OST file in the background during idle time. It needs more than 20 percent white space in the file.
- Compact Now. Delete unneeded items and empty Deleted Items first. Then follow the steps below.
How to Use Compact Now?
- Select File, Account Settings and Account Settings.
- Open the Data Files tab and select the data file. Then select Settings.
- For Exchange or Outlook.com accounts select the Advanced tab and then Outlook Data File Settings.
- Select Compact Now and then OK.
Compact Now needs several minutes. You do not need to exit Outlook after it finishes.
Related reading: how to fix the PST maximum size error and how to create a PST file in Outlook.
Frequently Asked Questions
Why does My PST File Remain Large After I Delete Mail?
Microsoft states deleting items does not always shrink a PST or OST file. Use Compact Now after you empty Deleted Items.
How do I Find Large Messages in Outlook?
Type messagesize:>5 mb in the search box and delete or move the results.
When does Outlook Compact a Data File on Its Own?
Outlook compacts in the background during idle time when the file has more than 20 percent white space.
Is a Size Limit Given for PST Files?
No. Microsoft gives no numeric limit on that page. It says only that providers limit mailbox size.
Key Takeaways
- Mailbox Cleanup finds large and old items.
- Empty Deleted Items before you compact.
- Outlook compacts on its own above 20 percent white space.
- Compact Now forces the step and needs several minutes.
- Microsoft gives no numeric size limit on this page.
Sources and References
Facts follow Microsoft Support.
- Reduce the size of your mailbox and Outlook Data Files (Microsoft Support) support.microsoft.com