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

We have a maximum mailbox size of 2GB and I’m currently using 700MB of it. However, when I look at my ost-file, I see that it is over 3GB in size.

What accounts for this difference and how can I make my ost-file smaller?

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 as well!

When you use Outlook 2013, the ost-file can actually be smaller than your mailbox.

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) 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, the People Pane 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 folders of additional mailboxes linked to your Exchange account.

Ost-files of Outlook Hotmail Connector, EAS and IMAP accounts

Aside from Exchange account, ost-files are also being used by Outlook Hotmail Connector accounts and in Outlook 2013, Exchange ActiveSync (EAS) and IMAP accounts also use ost-files.

In these cases, the ost-file is also mainly used for caching the mailbox data which is stored on the Outlook.com/Hotmail or IMAP servers. However, there can also be stored some unique data in there 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 mail server:

IMAP 2013 Hotmail Connector ActiveSync
Drafts * Tasks Drafts
Sent Items * Notes Notes
Trash/Deleted Items * Journal Journal
Junk E-mail *    
Calendar    
Contacts    
Tasks    
Notes    
Journal    

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

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.

For IMAP accounts in Outlook 2013, it is 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

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.

The ost-file can be easily 20-30% larger than the reported mailbox size when using Outlook 2010 or previous. This is due to some storage inefficiency of the ost-file and the way Exchange calculates the mailbox size (Exchange 2013 calculates it more accurately though).

When you use Outlook 2013, the ost-file can actually be up to 30% smaller than the reported mailbox size. This is because Outlook 2013 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 and are using Outlook 2007 SP2 or later, your ost-file can be as large as 10GB and you’d still won’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 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, you can use the Sync Slider to only cache more recent items rather than the entire mailbox. Note that this only applies to your mailbox.
  • 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 sub folders 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. This also applies to Outlook 2010 and previous which store the IMAP cache in a pst-file.

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 an IMAP pst-file or Hotmail ost-file



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