string ConfigNamespace = "http://schemas.microsoft.com/cdo/configuration/";
Correo.Fields.Add(ConfigNamespace + "sendusername", "nombreUsuario");
Correo.Fields.Add(ConfigNamespace + "sendpassword", "contraseña");
newMessage.Fields.Add(ConfigNamespace + "smtpauthenticate", "1");
//0:Perform no authentication. 1:Use the basic (clear text) authentication mechanism. 2:Use the NTLM authentication mechanism.