If you dig around in the EWS documentation, you’ll find this element: WebClientReadFormQueryString, which should give you a URL for an item in OWA. When Exchange Server 2013 shipped, we had some EWS developers who noticed this element was still giving out URLs in Exchange Server 2010’s OWA URL format, which no longer works with Exchange 2013. If you look in the Remarks for the WebClientReadFormQueryString article, you’ll see we‘ve added documentation to allow you to build URLs for 2013 by hand.
So – what happened? As Kristian describes it, Outlook Web Access in Exchange 2013 is a complete rewrite from the previous versions of OWA. This means a whole lot of new features (like Office Mail Apps), but it also means some features got left out. This article gives the highlights. In that article, you’ll note that Outlook Web App customization, as it existed in earlier versions, is not available in Exchange 2013. Part of this includes the old 2010 style URLs that could be used to create and access different types of items.
So – with Exchange 2013, you can use the format given in the documentation for WebClientReadFormQueryString to create a URL to view an existing email message. We do not have a similar URL format for other types of items, such as Appointments, Contacts, or Tasks. We also do not have a URL format for creation of items.