求助：使用SmtpClient通过465端口发送邮件失败

I’ve run into this exact issue before with Gmail and SmtpClient—port 465 can be tricky even when you set EnableSsl = true. Let’s break down the most common fixes:

1. Verify Gmail Account Security Settings #

• If you have 2-Step Verification (2FA) enabled on your Gmail account, you can’t use your regular password with SmtpClient. You need to generate an App Password instead:
  • Go to your Google Account > Security > App Passwords (only visible if 2FA is active).
  • Create a new app password for "Mail" and use that password in your SmtpClient configuration.
• Even without 2FA, Google no longer supports "Less secure app access"—so App Passwords are still the recommended approach for authenticated SMTP access.

2. Correct SmtpClient Configuration for Port 465 #

SmtpClient’s default SSL protocol might not align with Gmail’s requirements (Gmail mandates TLS 1.2 or higher). Try explicitly setting the SSL protocol:

using (var client = new SmtpClient("smtp.gmail.com", 465))
{
    client.EnableSsl = true;
    // Explicitly enforce TLS 1.2 (required by Gmail)
    client.SslProtocols = System.Security.Authentication.SslProtocols.Tls12;
    client.Credentials = new NetworkCredential("your-email@gmail.com", "your-app-password");
    
    var message = new MailMessage(
        "sender@gmail.com",
        "recipient@gmail.com",
        "Test Subject",
        "Test Body Content"
    );
    
    client.Send(message);
}

For older .NET versions where SslProtocols isn’t directly available on SmtpClient, set it globally first:

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;

3. Rule Out Firewall/Antivirus Blocking #

Port 465 is sometimes blocked by corporate firewalls or antivirus tools. Test sending from a different network (like home Wi-Fi) to confirm if this is the issue.

4. Switch to MailKit (Recommended) #

The built-in SmtpClient is obsolete in .NET Core/.NET 5+. MailKit is a more reliable alternative that handles SSL/TLS negotiation automatically, reducing configuration headaches:

using MailKit.Net.Smtp;
using MimeKit;

var message = new MimeMessage();
message.From.Add(new MailboxAddress("Sender Name", "sender@gmail.com"));
message.To.Add(new MailboxAddress("Recipient Name", "recipient@gmail.com"));
message.Subject = "Test Email via MailKit";
message.Body = new TextPart("plain") { Text = "This works seamlessly with port 465!" };

using (var client = new SmtpClient())
{
    client.Connect("smtp.gmail.com", 465, true); // True enables SSL
    client.Authenticate("your-email@gmail.com", "your-app-password");
    client.Send(message);
    client.Disconnect(true);
}

Content of the question originates from Stack Exchange, question author Alaa'