I get a runtime error trying to impresonate a user - code below

  Service = New ExchangeService()

        Service.Url = New Uri(ApplicationItems.ExchangeURL)
        Service.Credentials = New NetworkCredential(username, password)
        Service.ImpersonatedUserId = New ImpersonatedUserId(ConnectingIdType.SmtpAddress, emailAddress)

The error is telling you that what ever user you have used in

 Service.Credentials = New NetworkCredential(username, password)

Doesn't have Impersonation rights, Impersonation right need to granted to that user see https://msdn.microsoft.com/en-us/library/office/bb204095%28v=exchg.140%29.aspx

Cheers
Glen