How to get microsoft exchange server hostname, when I have only username

Use of 'ExchangeService' object from Microsoft.Exchange.WebServices is quiet comfortable to connect to exchange mailbox , where settings of the mail server are exposed using AutoDiscover Server.
In Rebex EWS object need to connect need to provide 'hostname'(min requirement). When I give hostname from email Id (user@hostname(.com) ) to connect I get 'Rebex.Net.NetworkSessionExceptionStatus.ServerProtocolViolation' exception with message "SOAP response expected" .