In Thunderbird (and I assume in many other clients, too) I have the option to choose between "SSL/TLS" and "STARTTLS".

As far as I understand it, "STARTTLS" means in simple words "encrypt if both ends support TLS, otherwise don't encrypt the transfer". And "SSL/TLS" means in simple words "always encrypt or don't connect at all". Is this correct?

Or in other words:

Is STARTTLS less secure than SSL/TLS, because it can fallback to plaintext without notifying me?