Exchange2015/Outlook 2013 Junk Filter

I have a user who is wanting to use the Outlook 2013 Junk Mail filters so he can mark emails as junk and have it no longer accept emails from that user or domain.
After reading the Tech Net article about Junk Mail I learn that I have to enable Exchange Cached Mode.
Once enable it starts complaining that the user is over his storage quota and he cannot send or receive until he deletes enough emails. The limit is set at 2GB and the user has an 8GB inbox according to the Exchange server. I have set all limits I can find in User Mailbox Settings on the server to either unlimited or at least double his usage.

Is there a way to set his offline storage limit to a larger size or alternatively allow junk/spam filtering that does not require Cached Mode?
I know I can archive the bulk of his mail but it would be easier on the user to keep it all un-archived as he sometimes needs instant access to his quotes, invoices, and orders.
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The most efficent solution is to archive / export out the bulk of that users message history and create an "offline" external .PST file.  Then reduce / compact the users "online" mail store.  You can then use Outlook to Open the independent .PST archive of the "offline" messages with the "online" exchange mail store.  If you don't disconnect the .PST file, it will open and be available each time Outlook is starts.  The user has immediate access to both "online" and "offline" messages.

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