Starting in June 2009, organizations can use Outlook 2003 to connect to Exchange Online Standard. Prior to this update, customers could connect to Exchange Online Standard via Outlook 2007 or Outlook Web Access (OWA). This update will not impact Exchange Online Dedicated customers, typically enterprise organizations with more than 5,000 end-users. Exchange Online Dedicated fully supports Outlook 2003, Outlook 2007, and OWA today.
Source:→ MOS Blog