"The remote certificate is invalid according to the validation procedure." using Gmail SMTP server

I know I am pretty late in this game, but I haven't seen an answer here pointing to the system.diagnostics logs for the TLS Stream.

Before you do any changes to your code, make sure you understand what the problem is about. The AuthenticationException is one of that very generic exception which does not tell much. To learn what's going under the hood edit the app.config file for your application (or create a new one) and make sure you have System.Net trace source enabled in the system.diagnostics section, for example:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <system.diagnostics>
    <trace autoflush="true" />
    <sharedListeners>
      <add name="file" initializeData="c:\network.log" type="System.Diagnostics.TextWriterTraceListener" />
    </sharedListeners>
    <sources>
      <source name="System.Net" switchValue="Verbose">
        <listeners>
          <add name="file" />
        </listeners>
      </source>
    </sources>
  </system.diagnostics>
</configuration>

Rerun your application and check the c:\network.log file. You should see there detailed information about your TLS (SSL) connection, for example:

System.Net Information: 0 : [12764] InitializeSecurityContext(credential = System.Net.SafeFreeCredential_SECURITY, context = f44368:535f958, targetName = localhost, inFlags = ReplayDetect, SequenceDetect, Confidentiality, AllocateMemory, InitManualCredValidation)
System.Net Information: 0 : [12764] InitializeSecurityContext(In-Buffers count=2, Out-Buffer length=0, returned code=OK).
System.Net Information: 0 : [12764] Remote certificate: [Version]
  V3

[Subject]
  CN=test
  Simple Name: test
  DNS Name: example.com

[Issuer]
  CN=Root CA
  Simple Name: Root CA
  DNS Name: Root CA

...

[Signature Algorithm]
  sha256RSA(1.2.840.113549.1.1.11)

[Public Key]
  Algorithm: RSA
  Length: 2048
  Key Blob: ....
System.Net Information: 0 : [12764] SecureChannel#38496415 - Remote certificate has errors:
System.Net Information: 0 : [12764] SecureChannel#38496415 -    Certificate name mismatch.
System.Net Information: 0 : [12764] SecureChannel#38496415 - Remote certificate was verified as invalid by the user.
System.Net Error: 0 : [12764] Exception in AppDomain#10923418::UnhandledExceptionHandler - The remote certificate is invalid according to the validation procedure..

Knowing what causes the problem, you should be able to resolve it or at least narrow your Google searches.

Adding this line worked for me. This in fact trusts all the certificates as mentioned here. However, this can be used primarily for troubleshooting. If this works for you, then it means that the certificate of the remote server is not added as a trusted certificate in your machine.

System.Net.ServicePointManager.ServerCertificateValidationCallback = new System.Net.Security.RemoteCertificateValidationCallback(RemoteServerCertificateValidationCallback);

Full code is

private void sendAMail(String toAddress, String messageBody)
        {
            String msg = "Sending mail to : " + toAddress;

            MailMessage mail = new MailMessage();
            mail.To.Add(toAddress);
            mail.From = new MailAddress("from@mydomain.com");
            mail.Subject = "Subject: Test Mail";
            mail.Body = messageBody;
            mail.IsBodyHtml = true;            

            //Added this line here
            System.Net.ServicePointManager.ServerCertificateValidationCallback = new System.Net.Security.RemoteCertificateValidationCallback(RemoteServerCertificateValidationCallback);
            SmtpClient smtp = new SmtpClient();

            smtp.Host = "myhostname.com";            
            smtp.Credentials = new System.Net.NetworkCredential("sender@sample.com", "");
            smtp.EnableSsl = true;
            smtp.Port = 587;            
            smtp.Send(mail);            
        }


private bool RemoteServerCertificateValidationCallback(object sender, System.Security.Cryptography.X509Certificates.X509Certificate certificate, System.Security.Cryptography.X509Certificates.X509Chain chain, System.Net.Security.SslPolicyErrors sslPolicyErrors)
{
    //Console.WriteLine(certificate);
    return true;
}

Below code worked for me in dotnetcore to bypass the ssl certificate

using (var httpClientHandler = new HttpClientHandler())
{                  
 httpClientHandler.ServerCertificateCustomValidationCallback = (message, cert, chain, errors) => true;
  using (var client = new HttpClient(httpClientHandler))
  {
  }
}

If you are getting this error while upload files to S3 using AWS SDK. Make sure that you have selected the correct region for S3.

If you have used the incorrect region. That will return such an error which is not meant for this error btw.

here is the solution that I decided to use.

        ServicePointManager.ServerCertificateValidationCallback = delegate (object s, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
        {
            string name = certificate.Subject;

            DateTime expirationDate = DateTime.Parse(certificate.GetExpirationDateString());

            if (sslPolicyErrors == SslPolicyErrors.None || (sslPolicyErrors == SslPolicyErrors.RemoteCertificateNameMismatch && name.EndsWith(".acceptabledomain.com") && expirationDate > DateTime.Now))
            {
                return true;
            }
            return false;
        };

The code from the accepted answer helped me to debug the issue. I then realized that the SN field of the certificate argument wasn't the same as what I thought was my SMTP server. By setting the Host property of the SmtpClient instance to the SN value of the certificate I was able to fix the issue.

use my function as diagnostic certificates issues - see screen

            System.Net.ServicePointManager.ServerCertificateValidationCallback = Function(s As Object,
                                                                                          cert As System.Security.Cryptography.X509Certificates.X509Certificate,
                                                                                          chain As System.Security.Cryptography.X509Certificates.X509Chain,
                                                                                          err As System.Net.Security.SslPolicyErrors)
                                                                                     Return True
                                                                                 End Function

Haven't seen anyone point out the obvious, that if you want to debug SSL issues safely then always use this approach:

#if DEBUG
    ServicePointManager.ServerCertificateValidationCallback += (sender, cert, chain, sslPolicyErrors) => true;
#endif

Even if you do accidentally commit your code, you'll still be OK it won't be compiled in Release mode.

Ran into this error running a dotnet core web application on an Amazon Linux EC2 virtual machine. Turns out we hadn't run sudo yum update to patch this server in a while, and the root/intermediate certificates installed on the machine were out of date. (Specifically, the issue started for us on 2021-09-30.)

We ran this command to update the certs and then restarted our dotnet process:

sudo yum update ca-certificates

AWS support further pointed us to this support article for similar issues: Why am I receiving a certificate expiration error for the Let's Encrypt certificate on my EC2 instance?