Over the size limit error message

[somedea]

We are upgrading from Office 2000 to Office 2007. We are runnng Exchange server 2003 SP2 Enterprise Edition. Since storage is cheap, up till now I have been instructed to place no limits on mailbox sizes.
Although I have no limits set on mailbox size, I'm getting an "over the limit" error message when I open Outlook 2007. Although my personal mailbox is not that big, I monitor a couple of other mailboxes, so I think they're all going into the local PST or OST file. I have 2 different PC's set up, one in cached mode and the other NOT in cached mode. Only the PC NOT running in cached mode gets this error message.
Why am I getting over the limit errors when I have no limits set on the server; AND if it's a local setting, where is it? Microsoft technet says it's in HKCU\Software\Policies\Microsoft\Office, but I don't have an Office key under Microsoft, only System Certificates and Windows.

 

[TechyMcSe2k]

Are there hard limits within the Exchange General/Storage Limits tab, for the actual AD account?

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[somedea]

They are set to "use the mailbox store defaults"

I think the problem may be with the upgrade from 2000. I'm reading that PST files were in ANSI format for 2000, and that has a file size limit of 2 GB. I'm going to try deleting all the PST files (after I back them up somewhere, of course) and then I'll recreate the user mail profile and see if I can get it change to unicode format. Then I'll see if I still have a problem.
Thanks!