I came across this great solution which can get you most of the way there with your public folder support for legacy SharePoint 2003 to Exchange Online migration scenarios. It's from a company named MessageOps (http://www.messageops.com/software/sharepoint-bridge) and the product is called SharePoint Bridge. What it effectively allows you to do is move messages from a designated Exchange Online mailbox into a SharePoint Online document library. Check it out at the link above.
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