I’m using Outlook 2010 and I’ve noticed that my ost-file for my Exchange account is much (much!) larger than it was when I was still using Outlook 2007.
Aside from my own mailbox, I also have the mailbox of my manager and a colleague added to Outlook as I am a delegate for them. It appears that Outlook 2010 fully caches these mailboxes now as well and not just the Calendar and Contacts.
Can I change Outlook 2010′s caching behavior back to the behavior of Outlook 2007?
Outlook 2010 indeed now fully caches secondary Exchange mailboxes as well and stores that cached data in the same ost-file as your own mailbox.
Outlook 2013 does this as well, but since Outlook 2013 compresses certain contents within the ost-file, the size increase may not be as dramatic as with Outlook 2010.
To revert back this behavior to the behavior of Outlook 2007, you must change/add the Registry key below while Outlook is closed;
Note: This Registry key can be used with Outlook 2007 as well to turn on caching of mail folders as well. For this see; Cache shared mail folders in Outlook 2007
New behavior and compacting the ost-file
Once you’ve changed the registry key and restarted Outlook, only the Calendar, Contacts, Tasks and Notes folders of the secondary mailbox will be cached.
When you are accessing a folder which is cached, the connection state in the right bottom corner will say “Connected”. When you are accessing a folder which isn’t cached, this connection state will say “Online”.
Your ost-file will shrink over time when Outlook is running and your computer is idle. As an alternative you can use the Compact Now feature or rename your ost-file to .old to completely rebuild your ost-file.