You need to specify correct port to use SSL. From SMTP protocol it’s usually 465. You can also use Smtp.DefaultSSLPort property.

using (Smtp client = new Smtp())
{
    client.Connect("smtp.gmail.com", 465, true);
    // which is equivalent to:
    // client.ConnectSSL("smtp.gmail.com");

To use StartTLS method first connect without SSL (otherwise you’ll get “Unrecognized command” error).

using (Smtp client = new Smtp())
{
    client.Connect("smtp.gmail.com", 25, false);
    // which is equivalent to:
    // client.Connect("smtp.example.org");
    client.StartTLS();

You can check if server supports StartTLS command with following code:

bool supportsTLS = client.SupportedExtensions().Contains(SmtpExtension.StartTLS);

ConnectSSL error:
The handshake failed due to an unexpected packet format.

StartTLS error:
5.5.1 Unrecognized command. u1sm39033912yhu.11

Thanks

@Hendrik

You are right smtp.Ehlo() is missing from the examples (after Connect and after StartTLS methods).
The latest version will not require those.

Edit:
Helo/Ehlo commands are no longer needed in the latest version.