The Implications of Outlook 2013 Changing OST Cache Behavior

When I wrote about my initial experiences of Outlook 2013 Preview on July 24, I remarked that the installation of Outlook 2013 forced a recreation of my Offline Storage file (OST). The new OST was much smaller than the older version used by Outlook 2010, a fact that seemed to be a good thing at the time even if the creation of OSTs en masse might generate a resource consumption problem for servers if you deployed Outlook 2013 to multiple users at one time.

The smaller OST has persisted since and, as I write, it’s about 600MB smaller than the Outlook 2010 version. Such a wonderful reduction warranted further investigation, so off I went to find out. has long been a good source of information about Outlook. Its piece on “All you need to know about Outlook 2013 Preview” said that Outlook 2013 uses a new compressed format for the OST which results in files up to 40% smaller than before. states that the reduction is gained because “Outlook now compresses several data fields/values in the ost-file which results in a much smaller ost-file”. These reports certainly match with my experience, so I was even more impressed at Microsoft’s achievement.

Then I went to Microsoft’s own “What’s new in Outlook 2013 Preview” page, where I discovered the same comment about new OST files being 40% smaller. But then I read all about “Cached Exchange Mode Improvements” to find that:

By default, if Cached Exchange Mode is enabled, Outlook 2013 Preview will only cache email messages from the last 12 months and remove anything older than 12 months from the local cache. Users can view messages that were removed from the local cache by scrolling to the end of an email list in a folder and clicking the message Click here to view more on Microsoft Exchange (see below). Users can also change how many email messages are kept offline. You, as the IT Administrator, can change the default age or enforce the age of email messages that are removed from the local cache by using Group Policy or the Office Customization Tool.”

Eureka! Although I have no doubt that Microsoft has made many wonderful tweaks to the Outlook 2013 OST file format, I suspect that the reduction in size that I’ve seen is heavily influenced by the change in caching behavior. Like many people who have used Exchange for more than a year, my mailbox is cluttered with items that are more than a year old. Outlook 2013 now ignores these items and doesn’t keep copies in the OST. This results in a smaller OST; it also means that I can’t access these items when working offline. Who’s to know when I need to look at a project report from 1995? Outlook 2013 clearly doesn’t think that this information is important, hence its eagerness not to cache the item.


It’s reasonable to assume that the change has been influenced by the greater availability of network connectivity currently enjoyed by most of the civilized world. After all, if you always have a network connection, you can always get back to the Exchange server to view the data that’s not cached in the OST (assuming of course that you ever want to look at old data). The same behaviour is seen whether you run on-premises or cloud versions of Exchange.

Network availability is certainly a valid influence on the discussion. It’s also true that humans don’t do a good job of cleaning mailboxes of the dross that accumulates there over time. I try my best to delete unwanted items as I deal with them but as soon as an item drops off the data visible in the opening screen displayed when Outlook opens the Inbox, it sinks into the quicksand of invisibility and is likely to become one of the thousands of items stored in OSTs but are never viewed from one year to the next.

“Helping” users to clean up their mailboxes by not caching items older than a year might be an intelligent tactic from the perspective of the Outlook development group. I have a nasty inkling that a different opinion might be held by users when they discover that they can’t get to data that they need because it’s just expired out of the cache. The same surprise is likely for administrators who might not even know that this change has been made until they receive the first report from the help desk. As Microsoft’s web page says, users can change the amount of data that they keep – it just doesn’t tell you that you have to go to Account Settings rather than Outlook Options to do this. When you get there, you’ll find that you can select caching for 1 month, 3 months, 6 months, 1 year, 2 years, and “all”. Administrators can also use the Outlook Customization Tool to enforce a setting across multiple clients.


On the positive side, even after I updated my cached Exchange mode setting to "all" to have Outlook 2013 revert to the previous method of caching, the size of my OST only increased by some 200MB, meaning that the size shrunk by some 400MB or roughly 35% when compared to the OST used by Outlook 2010. You have to take these figures with a pinch of salt because obviously the contents of my mailbox have changed since I started to use Outlook 2013 full-time on July 18. However, the change in the way that Outlook compresses some data fields in the OST seems to have had the desired effect, even if it means that the deployment of Outlook 2013 means that every OST has to be rebuilt.

Learn more: Outlook 2013 introduces hybrid cached mode (October 2012)

Change is wonderful. I just wish I wasn’t taken by surprise when settings change without warning!

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