使用 SmtpClient.ServicePoint.BindIPEndPointDelegate

http://msdn.microsoft.com/en-us/library/system.net.servicepoint.bindipendpointdelegate.aspx

 var smtpClient = new SmtpClient(...);
smtpClient.ServicePoint.BindIPEndPointDelegate = (servicePoint, remoteEndPoint, retryCount) =>
  new IPEndPoint( IPAddress.Parse( "YOUR LOCAL ADDRESS" ), 0 );