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Jul 232010

The OST file as most people will know is the client side “local cache” that the Outlook client creates.  Also caused cached mode Outlook, by many people.  A question was raised the other day which goes along the lines of :


If I have 5000 items in here, and they all get archived and turned into shortcuts, will the OST shrink?


At first I thought it wouldn’t shrink, but I was wrong.  Per the Microsoft article : http://support.microsoft.com/kb/289987/en-us, the OST will shrink automatically, when idle CPU is detected.  I tested this as follows :

I just inserted 5000 random messages in to my mailbox.. in online mode.

I then setup cached mode. 

The OST file is now  : 49.85 Mb

I then deleted ALL the items in the folder (shift deleted them), and waited a while.

The OST file is now : 54.8 Mb

I then purged the deleted items (by going into Recover Deleted Items, and clearing the items), and waited a couple of hours.

Now the OST is just 1.1 Mb


So the Microsoft article is proven to be correct.

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