It seems that your issue is connected with double hop issue:
http://blogs.msdn.com/b/mjeelani/archive/2004/12/07/275921.aspx
And it seems that the only secure way to enable credentials transfering from your client machine to SharePoint server and then to Exchange server is to enable Kerberos authentication in SharePoint and in Exchange.
http://support.microsoft.com/?id=832769
http://technet.microsoft.com/en-us/library/gg502594.aspx
Virto Calendar problem with "Authenticate as logged-on user" option in Exchange data source Print
Modified on: Fri, Jul 11, 2014 at 7:01 AM
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