使用MailKit发送邮件时出现535认证失败问题求助

问题场景 #

确认SMTP凭证无误，但调用client.Authenticate()时触发MailKit.Security.AuthenticationException，错误提示535: Authentication Failed，相关代码如下：

var message = new MimeMessage();
message.From.Add(new MailboxAddress("noreply", "noreply@xyz@gmail.com"));
message.To.Add(new MailboxAddress("xyz", "xyz@gmail.com"));
message.Subject = "Test Email";
message.Body = new TextPart("html")
{
    Text = "Test email sent successfully."
};

var client = new MailKit.Net.Smtp.SmtpClient();
try
{
    ServicePointManager.ServerCertificateValidationCallback += (sender, cert, chain, sslPolicyErrors) => true;
    client.SslProtocols = System.Security.Authentication.SslProtocols.Tls12;
    client.Connect("smtp.zeptomail.in", 587, false);
    client.Authenticate("emailapikey", password);
    client.Send(message);
    client.Disconnect(true);
}
catch (Exception e)
{
    Console.Write(e.Message);
}

排查及修复步骤 #

• 修正认证用户名：Zeptomail SMTP的认证用户名固定为apikey，而非emailapikey，修改认证代码：

  client.Authenticate("apikey", password);
  

• 启用STARTTLS加密：端口587为非SSL端口，需在连接后手动启用STARTTLS，否则未加密的连接会导致认证失败，在client.Connect()后添加：

  client.StartTls();
  

• 替换证书验证回调：MailKit不依赖ServicePointManager的证书验证机制，改用自身的回调方法：

  client.ServerCertificateValidationCallback = (sender, cert, chain, sslPolicyErrors) => true;
  

• 验证发件人地址权限：确保发件人邮箱地址已在Zeptomail平台完成域名验证或地址授权，未验证的地址可能触发后续发送失败。

修正后的完整代码 #

var message = new MimeMessage();
message.From.Add(new MailboxAddress("noreply", "noreply@xyz@gmail.com"));
message.To.Add(new MailboxAddress("xyz", "xyz@gmail.com"));
message.Subject = "Test Email";
message.Body = new TextPart("html")
{
    Text = "Test email sent successfully."
};

var client = new MailKit.Net.Smtp.SmtpClient();
try
{
    client.ServerCertificateValidationCallback = (sender, cert, chain, sslPolicyErrors) => true;
    client.SslProtocols = System.Security.Authentication.SslProtocols.Tls12;
    client.Connect("smtp.zeptomail.in", 587, false);
    client.StartTls();
    client.Authenticate("apikey", password);
    client.Send(message);
    client.Disconnect(true);
}
catch (Exception e)
{
    Console.Write(e.Message);
}

内容的提问来源于stack exchange，提问作者Ashok