Gmail SMTP secure connection issue when sending emails with C#

Other Questions Gmail
1

Having trouble with C# email sending functionality

I’m trying to create a simple email sender using C# that can send messages from one Gmail account to another. I’ve looked at several tutorials online but keep running into the same error message.

Here’s my current implementation:

using System;
using System.Net.Mail;
using System.Net;
using System.Windows.Forms;

namespace EmailSender
{
    public class MailService
    {
        public void SendMessage()
        {
            // Create email message
            MailMessage email = new MailMessage();
            email.From = new MailAddress("[email protected]");
            email.To.Add("[email protected]");
            email.Subject = "Test Email";
            email.Body = "This is a test message from C# application";
            email.CC.Add("[email protected]");
            
            // Configure SMTP client
            SmtpClient smtpServer = new SmtpClient("smtp.gmail.com");
            smtpServer.Port = 587;
            smtpServer.Credentials = new NetworkCredential("[email protected]", "password123");
            smtpServer.EnableSsl = true;
            smtpServer.DeliveryMethod = SmtpDeliveryMethod.Network;
            
            // Send email
            try
            {
                smtpServer.Send(email);
                MessageBox.Show("Email sent successfully!");
            }
            catch (Exception error)
            {
                MessageBox.Show("Failed to send: " + error.Message);
            }
        }
    }
}

I’ve tried different port numbers like 25, 465, and 587 but nothing works. The error message I keep getting says the server doesn’t support secure connections. Has anyone else faced this problem? What am I missing in my configuration?

2

first, check if 2-step verifcation is on in your Gmail. ya can’t get app passwords without it, probably why it’s failin. also, double-check your SMTP settings—time-out issues can cause weird SSL errors. try adding smtpServer.timeout = 20000 to allow more time to connect.

3

That secure connection error isn’t about authentication - it’s a TLS protocol mismatch. Gmail’s SMTP servers got stricter with security requirements, and .NET Framework’s default TLS settings don’t match what Gmail wants. Skip the App Password stuff for now and add ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12; at the start of your SendMessage method. This forces TLS 1.2, which Gmail requires. I hit this exact problem last year and wasted hours on authentication troubleshooting when TLS version was actually causing the “server doesn’t support secure connections” error. Get that fixed first, then worry about the App Password setup.

4

This isn’t an SSL problem - it’s Gmail’s authentication that’s blocking you. Google killed regular password support for SMTP and now forces you to use App Passwords for third-party apps. Here’s what you need to do: turn on 2FA for your Gmail account, then create an App Password just for your app. Swap out your regular password for that 16-character App Password in NetworkCredential. Your SSL settings and port 587 look fine, but Gmail won’t let you connect without proper auth. You can also try enabling ‘Less secure app access’ in your Google settings, but App Passwords are the way to go now.