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Tips & Tricks to Decrease, Manage or Reduce Outlook Mailbox (.ost & .pst) Size

Admin | How to's | 5 minutes read | Last updated on February 8th, 2023,

Many organizations today now prefer Outlook to exchange information over emails as it is known for most secure and reliable email application for desktop. The biggest and remarkable feature of MS Outlook is the way it stores and manage the mailbox with emails, calendar, contacts, notes, tasks, journals and other additional items.

MS Outlook enables to configure email account supporting SMTP, IMAP or POP3 protocol, this means that you can easily set up Gmail or Yahoo account on MS Outlook while companies prefer exchange server for email exchange.

Microsoft Outlook stores all the mailbox data on personal computer in PST file which is represented as Outlook.pst i.e. Outlook Data File, it is just a copy of mailbox on the server. Based on the protocol (IMAP or POP3) needs to download emails as the original copies may or may not be present on the server. In contrast, exchange server generate an OST file (Offline Storage Table) instead of PST file (Personal Storage Table), and it remains in sync with the server. Every single modification done in Outlook mailbox will also be reflected in mailbox available on the server or vice-versa. However, PST is the folder format that stores not only emails but also the other Outlook data. Moreover, the PST file size contains limit storage capacity, it means that single mailbox might have multiple attached PST files.

The more emails you receive the larger Outlook mailbox will expand. Large mailboxes can take long time to open and search and many email providers including Office 365 account, limit the maximum mailbox size. When you receive a message that mailbox is full and over its size limit, then deletion of folders and the message could help but if you store any information in Outlook.pst, then the data file may not shrink automatically.

Note: If you are using POP or IMAP account, all the information of email account including email messages, folders, calendar, contacts and task is stored in .pst Outlook data file format. If you are using O365, Outlook.com or Exchange then a copy of mailbox is stored in .ost Offline Outlook data file.

And in order to reduce the .pst file size, compress the data file. Offline .ost Outlook data files cannot be compacted easily.

For New Office Versions

There are multiple ways that can be applied to reduce the size of the mailbox.

  • You can make use of Mailbox Cleanup tool to view mailbox size and single stored folders.
  • The tool allows to search for older or very large messages that you want to delete or move to Outlook Data File (.pst).
  • Often, the cause of large mailbox is a large Deleted items folder. The tool provides the button to view deleted items folder size and empty the same.

Sometimes, Outlook use Conflict folder to store conflicted items. Conflicts take place when copy of an item is saved on email server and is out of sync with copy of an item that is stored on local computer. The Mailbox Cleanup tool can delete or discard these conflicts for you.

How to Run Mailbox Cleanup Tool

Go to File tab of Outlook.

Click Tools >> Mailbox Cleanup.

Select the options to compact Outlook data file folder, View Mailbox Size, Find items older than, Find items larger than, View Deleted Items Size, Empty Deleted Items folder, view Conflicts Size or Empty Conflicts to implement required task.

Opt Conversation Clean Up to eliminate redundant messages

In long email thread, many times several messages containing all same information as prior messages. Conversation Clean Up can automatically remove those redundant messages.

Search for large messages

Search for larger messages than specific size from Outlook Search box. Bring cursor in the search box and enter: messagesize:> 5 mb. This will find all messages greater or larger than 5 MB. You can delete either all the message or some messages or can also transfer them to another .pst Outlook data file.

Reduce Outlook Data File Size (OST or PST)

If you remove items from PST Outlook Data File, the file will then automatically compressed in the background when you are not making use of computer when running MS Outlook. Also, you can do the manual process and start process to compact file immediately. It might take several minutes to compact Outlook data file (.pst).

  1. Delete unnecessary items and empty Deleted Items folder.
  2. Go to File >> Click on Account Settings >> Account Settings.
  3. From Data Files tab, click the required data file to compact and click Settings.
  4. Click Advanced tab >> Select Outlook Data File Settings.
  5. In Outlook Data File Settings dialog, click Compact Now and click OK.

Note: Once the Outlook Data File (.pst) get compacted, then you do not have to exit from Outlook.

Reduce Offline Outlook Data File Size (.ost)

Various email accounts make use of Offline Outlook Data File to save data. The .ost file is very much similar to .pst with just one exception. One cannot import from or export to an Offline Outlook data file.

  1. Go to File >> Click Account Settings >> Select Account Settings.
  2. Click Data Files tab, click the required data file to compress and click Settings.
  3. Click Advanced tab >> Select the option Outlook Data File Settings.
  4. Outlook Data File Settings dialogue box will appear, click on Compact Now and click OK.
  5. Note: No need to exit from Outlook after compacting Offline Outlook Data File (.ost).

Note: Follow the same process for Office 2010 and compact the Outlook mailbox file size.

For Office 2007

Managing Mailbox Size

Use Mailbox Cleanup Tool to view mailbox size and single folders. Also, archive older items to an archive Personal Folders file (.pst) and empty Deleted Items folder.

From Tools menu >> click Mailbox Cleanup.

Select the required options for compaction: View Mailbox Size, Find items older than, Find items larger than, View Deleted Items Size, Empty Deleted Items folder, View Conflicts Size, or Empty Conflicts to perform required task.

Reduce Personal Folders (.pst) file size

If you remove items from Personal Folder .pst file, the file is automatically compressed in the background when you are not using computer and Outlook is not running. If you want to immediately reduce the file size you can force a compaction. The procedure that compact the data could take several minutes.

  1. From File menu >> Click on Data File Management.
  2. Click on the required data file to compact and click Settings.
  3. Click the option Compact Now.

Note: After compacting .pst file, do not exit from MS Outlook.

Reduce Offline Folder (.ost) file size

Reducing .ost file size could improve the performance if you have removed large number of items. The process mentioned below does not remove any item that are stored on the server.

  1. Discard the items that is no longer needed and empty Deleted Items folder.
  2. From Tools menu, click on Account Settings.
  3. Choose MS Exchange Server from the list and click Change.
  4. Click on More Settings.
  5. Click Advanced tab and click Offline Folder File Settings.
  6. Click the option Compact Now.

Note: No need to exit from Outlook after compacting .ost file.

Still if you have OST files in a large number, then make use of OST Converter Tool and convert OST Offline Outlook Data File to number of format without any trouble.