The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required

Question

I have a Problem, when i m sending a mail through this code then error is occured that "The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required. "

and my code is :

protected void ImageButton1_Click(object sender, ImageClickEventArgs e)
{

    MailMessage mail = new MailMessage();
    mail.To.Add("[email protected]");
    mail.From = new MailAddress("[email protected]");
    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 = 25;
    smtp.UseDefaultCredentials = true;
    smtp.Credentials = new System.Net.NetworkCredential("[email protected]", "xyz");
    smtp.EnableSsl = true;
    smtp.Send(mail);


    txt_cname.Focus();
    txt_cname.Text = "";
    txt_addwithno.Text = "";
    txt_conperson.Text = "";
    txt_email.Text = "";
    txt_description.Text = "";

}

Answer 1

Google may prevent an application from accessing your account
Here you can enable applications to access google account with your credentials:

https://accounts.google.com/DisplayUnlockCaptcha