Getting an error when trying to connect to SMTP server "199.201.221.19"

Question

I am using the following code and getting an error when trying to connect to SMTP server. I am not giving any authentication credentials to access that.

var client = new SmtpClient()
await client.ConnectAsync("199.201.221.19", 587, false);
client.Send(mail);

Host:199.201.221.19 Port:587 Error: System.Security.Authentication.AuthenticationException: The remote certificate is invalid according to the validation procedure. What is the actual issue and what is the possible solution for this?

Answer 1

The error refers to an SSL/TLS certificate validation error. It means that your server's SSL certificate does not verify using .NET's standard Root CA certificates.

Even though you have specified useSsl as false in your ConnectAsync() method call, MailKit will still toggle into TLS-mode if the server supports STARTTLS. The useSsl parameter is really only useul in telling MailKit whether or not the port requires SSL from the very start (as in ssl-wrapped ports such as 993 for IMAP/S, 995 for POP/S and 465 for SMTP/S).

You have 2 options for fixing this problem:

1. Follow the directions for this error in the FAQ: https://github.com/jstedfast/MailKit/blob/master/FAQ.md#InvalidSslCertificate
2. use a different ConnectAsync method that allows you to specify not to try STARTTLS: client.ConnectAsync (host, port, SecureSocketOptions.None);