User with 6GB mailbox is ending up with 20GB OST file

We are running SBS 2003 which uses Exchange 2003, users mailbox size is 6GB but hes having problems because his outlook 2003 OST file is 20GB for some reason, I deleted the OST file and let outlook re-cache everything but its ended up as 20GB again...

They do use public folders extensively but I didnt think these get cached by outlook?

Any ideas appreciated! :)
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I think this is because the mailbox in the exchange store uses Single Instance Storage and in an OST this feature can't be used. The maximum size for Unicode OST files is 20GB. Maybe an idea is to archive a part of his mailbox to an archive .PST so his mailbox on the server is smaller and thus the resulting OST file will be smaller.
Larry LarmeuPrincipal ConsultantCommented:
1.Delete any items that you do not want to keep, and then empty the Deleted Items folder.
2.On the Tools menu, click Account Settings.
3.In the list, select Microsoft Exchange Server, and then click Change.
4.Click More Settings.
5.Click the Advanced tab, and then click Offline Folder File Settings.
6.Click Compact Now.
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Corp_JonesAuthor Commented:
Thanks for the quick responses, I had no idea about the single instance storage which would now make sense, I'll also get the user to try the compact now to see if that makes a difference, if not I guess the only option to create a newer format OST file is a new version of outlook...
Corp_JonesAuthor Commented:
Ah ha, I think I need to edit the exchange email account and choose the "more settings" button.  On the "advanced" tab, you have to have "Download Public Folder Favorites" un checked.

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Do you know if the user has multiple mailboxes open? Last time i seen those symptoms was when the end user had 4 other mailboxes open in Outlook from the exchange server. Although the OST file will have the primary account name, if they have open other mailboxes (Accounts, more settings, advanced) the file will cache the other mailboxes, making the OST file larger than it would normally be.
forgot to mention if the user is running Outlook 2010, instead of adding additional mailboxes from the primary account, you can add each mailbox as a seperate account to keep ost file size down.
Corp_JonesAuthor Commented:
Sorry guys what I posted was only thing that worked, i'll accept this for future ref by other users.
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