Microsoft has confirmed to Neowin that the upcoming version of Outlook 2013 for Windows RT users will be confined to the desktop, and has not been designed as a new-style Modern UI app.
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Outlook 2013 rt
A new post on Microsoft's official Surface Twitter account claims that the upcoming Outlook 2013 RT client for Windows RT will in fact be "a Windows Store app" and not a standard desktop app.
Microsoft has offered up some more information on the upcoming and highly anticipated Outlook 2013 RT client, which will be released for Windows RT devices later in 2013.