public MailMessage(string from, string to)
{
if (from == null)
throw new ArgumentNullException(nameof(from));
if (to == null)
throw new ArgumentNullException(nameof(to));
if (from == string.Empty)
throw new ArgumentException(SR.Format(SR.net_emptystringcall, nameof(from)), nameof(from));
if (to == string.Empty)
throw new ArgumentException(SR.Format(SR.net_emptystringcall, nameof(to)), nameof(to));
_message = new Message(from, to);
if (NetEventSource.IsEnabled) NetEventSource.Associate(this, _message);
}