无法从C＃发送电子邮件

Question

我正在尝试使用C＃发送电子邮件，如下所述，

System.Net.Mail.MailMessage mail = new System.Net.Mail.MailMessage();
mail.From = new System.Net.Mail.MailAddress("fakeemail@gmail.com");
SmtpClient smtp = new SmtpClient();
smtp.Port = 587;
smtp.EnableSsl = true;
smtp.DeliveryMethod = SmtpDeliveryMethod.Network;
smtp.UseDefaultCredentials = false;
smtp.Credentials = new NetworkCredential(mail.From.Address, "fakepassword");
smtp.Host = "smtp.gmail.com";

//recipient
mail.To.Add(new MailAddress("fakeemail@gmail.com"));

mail.IsBodyHtml = true;
string st = "Test";

mail.Body = st;
smtp.Send(mail);

但是它给了我以下错误，我不知道要解决此错误该问题的解决方案是什么？

<string xmlns:i="http://www.w3.org/2001/XMLSchema-instance"
xmlns="http://schemas.microsoft.com/2003/10/Serialization/"
i:nil="true"/>