OST File Does Not Match Mailbox Size in Exchange Server

My mailbox size on the server is 60MB. And when I see its size in MS Outlook by right-clicking mailbox name to select Properties and then Folder Size, it is 60 MB. But when I see the size of items in list, it is around 4MB. In fact, I checked size of all folders separately in MS Outlook but the size is never even close to 60GB. I performed these actions on Outlook versions 2000 and 2003, and Exchange version 5.5. I am really puzzled as my Outlook OST File Does Not Match Mailbox Size in Exchange Server. Can someone tell me a way to reduce mailbox size?

Looking at this query, we have a deeper understanding of the problem and we will also include the ways to resolve the problem by making OST file size smaller. Usually, the mailbox does not takes up the big part of Outlook OST file but it can have much more contents even bigger in size. In Outlook 2013 or 2016 version, OST file may be really smaller than mailbox.

Mailbox Size VS. OST File Size

OST file size and mailbox size can vary a lot. The size of OST file does not match with Exchange server's mailbox size even after all extra caching is disabled and "This Computer Only" folders are empty. It is normal for an OST file to be 20-30 % bigger than reported mailbox size in Outlook 2010 or below versions. The reason for this is storage incapability of OST file and the way Exchange measures size of mailbox. However, Exchange 2013 has a better potential of performing precise calculations.

Outlook can be approx 30% smaller compared to the reported mailbox size in Outlook 2013 or 2016 version. This is for a fact that Outlook new versions reduce many streams of text while saving in .ost file format. This can be compared to zipping a hefty text file. Therefore, a smaller OST file is obtained.

Compressing an Outlook OST file is not a cakewalk. Moreover, it is not required usually. But, if one is employing Outlook 2007 SP2 or higher version on a modern system then, OST file may be as big as 10GB and one would still not observe a performance hit. So, letting it grow big unwantedly is not the solution too. Now, these following methods and compacting ost file can be equipped to reduce OST file size.

If your mailbox stores larger number of files or items in single mailbox or if your mailbox is too large then Outlook freezes when you open or send a mail or Outlook not responding.

1. Recreating OST File :

One can prefer to let Outlook recreate .ost file than compressing it or awaiting its automatic compression after the clean up takes place. For this, rename the file with .old when Outlook is exit. Doing this will recache mailbox in a new Ost file as Outlook is restarted.

2. Sync Filters :

In order to figure out what gets synchronized and what gets saved in OST file, in case folders are large, user has to set the Synchronization Filters.

3. Sync Slider :

Make use of Sync Slider in order to cache more recent items instead of recaching entire mailbox in Outlook 2013 and 2016 versions.

4. Disabling Shared Folder's Caching :

User can disable the function to cache shared folders normally or not to cache shared mail folder in case user does not requires offline access to shared folders, and if the access is not very frequent.

5. Disabling Public Folder's Favorites Caching :

Make use of Sync SliUser can select the disable feature to cache Public Folder Favorites just like the above shared folders caching mechanism.

6. Erase few Public Folder's Favorites :

In order to minimize the quantity of data of being cached, erase few Public Folder Favorites or rather just choose some subfolders, instead of disabling the options all at once.

7. Duplication of Gmail :

Items can be copied because of the folders one is subscribed to, in a Gmail account. This is applicable on Outlook 2010 and below versions that save IMAPcache in a PST file.

The Bottom Line

Since, Outlook is used by a huge number of people, it is important to resolve every single problem related to it. So, in this content, we have covered all relevant information and general methods user can look upon in case Outlook OST file does not match mailbox size in Exchange server. But, if you are not able to fix your issues using above techniques, it is advisable to repair corrupt data file and compact OST file in Outlook. Hope the shared information will be of beneficial for the users unanimously.