Как включить расширения в URI тел:

это тоже сработает ..

string your_id = "your_id@gmail.com";
string your_password = "password";
try
{
   SmtpClient client = new SmtpClient
   {
     Host = "smtp.gmail.com",
     Port = 587,
     EnableSsl = true,
     DeliveryMethod = SmtpDeliveryMethod.Network,
     Credentials = new System.Net.NetworkCredential(your_id, your_password),
     Timeout = 10000,
   };
   MailMessage mm = new MailMessage(your_iD, "recepient@gmail.com", "subject", "body");
   client.Send(mm);
   Console.WriteLine("Email Sent");
 }
 catch (Exception e)
 {
   Console.WriteLine("Could not end email\n\n"+e.ToString());
 }