Most likely your POP3 server certificate is self-signed or you are using incorrect host name to connect.
Host name must match the name on certificate: for example pop3.example.com and example.com may point to the same server, but certificate is issued only to pop3.example.com and this is the address you should use.
You can ignore this error:
client.ServerCertificateValidate + (sender, e) => { e.IsValid = true; };
client.ConnectSSL("pop3.example.com");
You can find more details here:
https://www.limilabs.com/blog/the-remote-certificate-is-invalid-according-to-the-validation-procedure