Certificate Validation Error

Oct 13, 2025 by talynt.org 17 views

Introduction

In this article, we will explore how to use the MailKit library to create an SMTP client that connects to a custom SMTP server using the StartTLS protocol. The SMTP server will be using a self-signed certificate, which can be a common scenario in development environments or when working with internal mail servers. We will also discuss the importance of using a secure connection when sending emails and how to handle self-signed certificates in .NET.

What is StartTLS?

StartTLS is a protocol that allows a client to upgrade an existing insecure connection to a secure connection using the Transport Layer Security (TLS) protocol. This is particularly useful when connecting to a mail server that only supports insecure connections, but you want to ensure that your email is sent securely.

What is a Self-Signed Certificate?

A self-signed certificate is a certificate that is signed by the same entity that created it. This means that the certificate is not issued by a trusted Certificate Authority (CA), but rather by the organization itself. Self-signed certificates are often used in development environments or when working with internal systems, as they can be easily created and managed.

Using MailKit with StartTLS and Self-Signed Certificates

To use MailKit with StartTLS and self-signed certificates, we need to create an instance of the SslProtocols enum and pass it to the SslProtocols parameter of the SmtpClient constructor. We also need to specify the CertificateValidationCallback delegate to handle the validation of the self-signed certificate.

Here is an example of how to create an instance of the SmtpClient class using MailKit:

using (var client = new SmtpClient(new ProtocolLogger(Console.OpenStandardOutput())))
{
    client.Connect("smtp.example.com", 587, SslProtocols.Tls);
    client.Authenticate("username", "password");
    client.Send(message);
    client.Disconnect(true);
}

However, in our case, we need to use a self-signed certificate, so we need to create an instance of the SslProtocols enum and pass it to the SslProtocols parameter of the SmtpClient constructor. We also need to specify the CertificateValidationCallback delegate to handle the validation of the self-signed certificate.

Here is an example of how to create an instance of the SmtpClient class using MailKit and a self-signed certificate:

using (var client = new SmtpClient(new ProtocolLogger(Console.OpenStandardOutput())))
{
    client.Connect("smtp.example.com", 587, SslProtocols.Tls | SslProtocols.Ssl3);
    client.Authenticate("username", "password");
    client.Send(message);
    client.Disconnect(true);
}

However, in our case, we need to use a custom SMTP server, so we need to create an instance of the SmtpServer class and pass it to the SmtpClient constructor.

Here is an example of how to create an instance of the SmtpServer class:

var server = new SmtpServer("smtp.example.com", 587);
server.Certificate = new X509Certificate2("path/to/self-signed/certificate.pfx", "password");
server.Start();

Handling Self-Signed Certificates

When using a self-signed certificate, the MailKit library will throw an exception if the certificate is not trusted. To handle this, we need to create a custom CertificateValidationCallback delegate that will validate the self-signed certificate.

Here is an example of how to create a custom CertificateValidationCallback delegate:

var callback = (sender, certificate, chain, sslPolicyErrors) =>
{
    if (certificate.Subject == "CN=smtp.example.com")
    {
        return true;
    }
    return false;
};

We can then pass this delegate to the CertificateValidationCallback parameter of the SmtpClient constructor:

using (var client = new SmtpClient(new ProtocolLogger(Console.OpenStandardOutput())))
{
    client.Connect("smtp.example.com", 587, SslProtocols.Tls | SslProtocols.Ssl3);
    client.CertificateValidationCallback = callback;
    client.Authenticate("username", "password");
    client.Send(message);
    client.Disconnect(true);
}

Conclusion

In this article, we have explored how to use the MailKit library to create an SMTP client that connects to a custom SMTP server using the StartTLS protocol. We have also discussed the importance of using a secure connection when sending emails and how to handle self-signed certificates in .NET. By following the examples provided in this article, you should be able to create a custom SMTP client that uses StartTLS and self-signed certificates.

Example Use Cases

Here are some example use cases for the code provided in this article:

Development Environment: In a development environment, you may want to use a self-signed certificate to test your email sending code. By following the examples provided in this article, you can create a custom SMTP client that uses StartTLS and self-signed certificates.
Internal Mail Server: If you are working with an internal mail server that uses a self-signed certificate, you can use the code provided in this article to create a custom SMTP client that connects to the server using StartTLS.
Testing Environment: In a testing environment, you may want to simulate a mail server that uses a self-signed certificate. By following the examples provided in this article, you can create a custom SMTP client that uses StartTLS and self-signed certificates.

Troubleshooting

Here are some common issues that you may encounter when using the code provided in this article:

Certificate Validation Error: If you encounter a certificate validation error, it may be because the self-signed certificate is not trusted. To resolve this issue, you can create a custom CertificateValidationCallback delegate that will validate the self-signed certificate.
Connection Timeout: If you encounter a connection timeout error, it may be because the mail server is not responding. To resolve this issue, you can increase the connection timeout value or use a different mail server.
Authentication Error: If you encounter an authentication error, it may be because the username or password is incorrect. To resolve this issue, you can verify the username and password with the mail server administrator.

Conclusion

Q: What is the difference between StartTLS and SSL/TLS?

A: StartTLS is a protocol that allows a client to upgrade an existing insecure connection to a secure connection using the Transport Layer Security (TLS) protocol. SSL/TLS is a protocol that provides secure communication between a client and a server. While both protocols provide secure communication, StartTLS is specifically designed for use with insecure connections, whereas SSL/TLS is designed for use with secure connections.

Q: Why do I need to use a self-signed certificate with MailKit?

A: You need to use a self-signed certificate with MailKit when you are working with a custom SMTP server that uses a self-signed certificate. Self-signed certificates are often used in development environments or when working with internal systems, as they can be easily created and managed.

Q: How do I create a self-signed certificate with MailKit?

A: To create a self-signed certificate with MailKit, you can use the X509Certificate2 class to create a new certificate. You can then use the CertificateValidationCallback delegate to handle the validation of the self-signed certificate.

Q: What is the `CertificateValidationCallback` delegate?

A: The CertificateValidationCallback delegate is a delegate that is used to handle the validation of a certificate. When a certificate is presented to the client, the delegate is called to determine whether the certificate is valid or not.

Q: How do I handle self-signed certificates with MailKit?

A: To handle self-signed certificates with MailKit, you need to create a custom CertificateValidationCallback delegate that will validate the self-signed certificate. You can then pass this delegate to the CertificateValidationCallback parameter of the SmtpClient constructor.

Q: What are some common issues that I may encounter when using MailKit with StartTLS and self-signed certificates?

A: Some common issues that you may encounter when using MailKit with StartTLS and self-signed certificates include certificate validation errors, connection timeouts, and authentication errors.

Q: How do I troubleshoot certificate validation errors with MailKit?

A: To troubleshoot certificate validation errors with MailKit, you can create a custom CertificateValidationCallback delegate that will validate the self-signed certificate. You can then pass this delegate to the CertificateValidationCallback parameter of the SmtpClient constructor.

Q: How do I troubleshoot connection timeouts with MailKit?

A: To troubleshoot connection timeouts with MailKit, you can increase the connection timeout value or use a different mail server.

Q: How do I troubleshoot authentication errors with MailKit?

A: To troubleshoot authentication errors with MailKit, you can verify the username and password with the mail server administrator.

Q: Can I use MailKit with other protocols such as POP3 or IMAP?

A: Yes, you can use MailKit with other protocols such as POP3 or IMAP. However, you will need to use a different class such as Pop3Client or ImapClient to connect to the server.

Q: Is MailKit compatible with .NET Core?

A: Yes, MailKit is compatible with .NET Core. You can use MailKit with .NET Core to create a custom SMTP client that uses StartTLS and self-signed certificates.

Q: Can I use MailKit with other programming languages such as Java or Python?

A: Yes, you can use MailKit with other programming languages such as Java or Python. However, you will need to use a different library or framework to connect to the mail server.

Conclusion

In conclusion, using MailKit with StartTLS and self-signed certificates can be a complex task, but by following the examples provided in this article and troubleshooting common issues, you should be able to create a custom SMTP client that uses StartTLS and self-signed certificates. Remember to handle self-signed certificates by creating a custom CertificateValidationCallback delegate, and troubleshoot common issues such as certificate validation errors, connection timeouts, and authentication errors.