How to access Mail Inbox Item from Exchange Server 2013 using OWA Plugin OAuth Token

I have to tried to access mail inbox item from exchange server 2013 using OWA plugin Identity Token but I am getting authentication error, anyone has resolve this type of issue please share with me.

OWA Plugin Code: function getUserIdentityTokenCallback(asyncResult) { var token = asyncResult.value; //token Passing to Exchange Server 2013 for atuthentication... } var _mailbox = Office.context.mailbox; _mailbox.getUserIdentityTokenAsync(getUserIdentityTokenCallback);

Exchange Server Code:

ExchangeService exchange_service = new ExchangeService(ExchangeVersion.Exchange2013);

exchange_service.HttpHeaders.Add("Authorization", "Bearer " + token);
exchange_service.AutodiscoverUrl(EmailAddress);
EmailMessage emailItem = EmailMessage.Bind(exchange_service, new ItemId(emailtokenid));

Error: {"The request failed. The remote server returned an error: (401) Unauthorized."}