Opportunistic encryption between endpoints doesn't necessarily require STARTTLS. That is merely a current convention. It could change in future, with the development or invention of some new crypto paradigm. Case in point: opportunistic IPSEC. Okay that wasn't the answer either, but it is an alternative in principle, albeit not in practice today for global SMTP.
Point is, I think it's naive and unwise to assume that the crypto upgrade/wrapper is and always shall be TLS and then to build this assumption into standards in a manner that isn't future compatible. We are due a great many innovations in end-to-end crypto over the next few decades. For example, I'm hoping that new and interesting uses will be found for DANE. Maybe we can even bring back network-layer opportunistic encryption.
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Opportunistic encryption between endpoints doesn't necessarily require STARTTLS. That is merely a current convention. It could change in future, with the development or invention of some new crypto paradigm. Case in point: opportunistic IPSEC. Okay that wasn't the answer either, but it is an alternative in principle, albeit not in practice today for global SMTP.
Point is, I think it's naive and unwise to assume that the crypto upgrade/wrapper is and always shall be TLS and then to build this assumption into standards in a manner that isn't future compatible. We are due a great many innovations in end-to-end crypto over the next few decades. For example, I'm hoping that new and interesting uses will be found for DANE. Maybe we can even bring back network-layer opportunistic encryption.