What is stored in the ost-file and why is it larger or smaller than my mailbox?

Looking at the size of your ost-file isn’t really representative for the size of your mailbox.

Of course, in most cases, your own mailbox does make up the large part of the ost-file but there is much more stored in there which can be quite big or even bigger than your own mailbox as well!

The reverse can be true as well; Your ost-file can be smaller than your mailbox.

What is being stored in an ost-file also depends on whether you are using an Exchange or IMAP account.

What is stored in the ost-file of an Exchange account?

First of all, the ost-file of an Exchange account is only a cache of data which is stored on the Exchange server(s).

The main purpose of this cache is to reduce network traffic, reduce the payload on the Exchange server(s), improve Outlook performance, and provide offline availability.

In addition, it acts as a foundation for several features in Outlook which couldn’t be used otherwise such as Instant Search, and Conversation View.

In general, the following is stored in the ost-file:

• Your mailbox data (mail, calendar, contacts, tasks, notes and journal folders).
• Hidden items and other “overhead” as found in a pst-file.
• Public Folder Favorites.
• Shared Calendars.
• Shared Mailboxes linked to your Exchange account.

What is stored in the ost-file of an IMAP account?

In Outlook 2013, 2016, 2019, 2021 and Microsoft 365, IMAP accounts use ost-files as well.

In this case, the ost-file is also mainly used for caching the mailbox data which is stored on the IMAP mail server.

However, the ost-file could additionally contain some unique data which isn’t being synched with the mail server. These folders are marked with “This computer only”.

The following folders do not sync with the IMAP mail server:

• Drafts*
• Sent Items*
• Trash/Deleted Items*
• Junk E-mail*
• Calendar
• Contacts
• Tasks
• Notes
• Journal

* This applies only to some IMAP servers for which Outlook cannot automatically determine the special folders. For more information see: Setting the Sent Items folder for IMAP accounts in Outlook 2013, 2016, 2019, 2021, and Microsoft 365.

If you store data in these folders, it is recommended to make backups of these folders itself via an Export rather than a backup of the ost-file. This is because, unlike pst-files, ost-files can’t always be directly restored without the use of 3rd party recovery or conversion tools such as Stellar Converter for OST.

For IMAP accounts in Outlook 2013 and later, it is therefor recommended to set an additional pst-file as the default data store for the non-mail folders. To do this see: Don’t risk losing your Contacts and Calendar when using IMAP in Outlook 2013, 2016, 2019, 2021, or Microsoft 365.

Mailbox size vs. ost-file size

You probably now see that the mailbox size and the size of the ost-file can be quite far apart.

However, even when you would disable all the additional caching and store nothing in the “This computer only” folders, the size of the ost-file could be quite different from the size of the mailbox. This especially applies to Exchange accounts.

When you cache your entire Exchange mailbox, the ost-file can actually be up to 30% smaller than the reported mailbox size. This is because Outlook compresses certain streams of text when storing it in the ost-file. You could compare this to zipping a large txt-file; it allows for quite some compression and thus results in a smaller ost-file.

Reducing the size of the ost-file

Reducing the size of the ost-file isn’t always that easy and not always needed either. If you are on a reasonably modern computer with a fast SSD drive, your ost-file can be as large as 20GB and you’d still wouldn’t notice a performance hit.

Nonetheless, letting it grow unnecessarily large isn’t the way either. To reduce the size of the ost-file, you can use the following methods:

• Ost-file recreation
  After a major cleanup, you can choose to let Outlook recreate the ost-file instead of compacting it or awaiting automatic compression. To do this, rename the ost-file to .old when Outlook is closed. Upon restarting Outlook, your mailbox will be recached in a new ost-file.
• Sync Filters
  When you have some large folders, you can set Synchronization Filters to determine what gets synchronized and stored in the ost-file.
• Sync Slider
  In Outlook 2013 and later, you can use the Sync Slider to only cache more recent items rather than the entire mailbox. Note that for Outlook 2013 this only applies to your own mailbox and not to any additional shared mailboxes you may have connected. For Outlook 2016, 2019, 2021 and Microsoft 365, the sync slider applies to all cached mail folders.
• Disable caching of shared folders
  If you don’t need off-line access to shared folders and don’t regularly access them either, you can disable the option to cache shared folders in general or not to cache shared mail folders.
• Disable caching of Public Folders Favorites
  Similar to caching of shared folders, you can choose to disable the option to cache Public Folder Favorites.
• Remove some Public Folder Favorites
  Rather than disabling the option altogether, you could also remove some Public Folder Favorites or only select a few subfolders instead to reduce the amount of data of being cached.
• Gmail duplication
  When you use a Gmail account, items could be duplicated due to the folders you are subscribed to.

Note: If you want to reduce the file size of the ost-file because your disk is filling up, you can also consider moving it to another disk. For instructions on how to do this see: Moving the ost-file of an IMAP account.