可能重复：
SMTP 服务器需要安全连接或客户端未通过身份验证。服务器响应是：5.5.1 Authentication Required
我在运行此代码时遇到问题，然后出现错误“SMTP 服务器需要安全连接或客户端未通过身份验证。服务器响应为：5.5.1 需要身份验证。”
我的代码是：
代码
protected void ImageButton1_Click(object sender, ImageClickEventArgs e)
{
MailMessage mail = new MailMessage();
mail.To.Add("info@msslindia.com");
mail.From = new MailAddress("info@msslindia.com");
string body = "<table><tr><td>Company Name:</td><td>" + txt_cname.Text + "</td></tr><tr><td>Address With No.:</td><td>" + txt_addwithno.Text + "</td></tr><tr><td>Contact Person:</td><td>" + txt_conperson.Text + "</td></tr><tr><td>Email Id</td><td>" + txt_email.Text + "</td></tr><tr><td>Description</td><td>" + txt_description.Text + "</td></tr></table>";
mail.Body = body;
mail.IsBodyHtml = true;
SmtpClient smtp = new SmtpClient();
smtp.Host = "smtp.gmail.com";
smtp.Port = 587;
smtp.Credentials = new System.Net.NetworkCredential("info@msslindia.com", "12345");
smtp.EnableSsl = true;
smtp.Send(mail);
txt_cname.Focus();
txt_cname.Text = "";
txt_addwithno.Text = "";
txt_conperson.Text = "";
txt_email.Text = "";
txt_description.Text = "";
}