It's not possible to know if STARTTLS will be used before you send a message. Because the sending system will only use STARTTLS if it's advertised during delive

It's not possible to know if STARTTLS will be used before you send a message. Because the sending system will only use STARTTLS if it's advertised during delivery. Unless both the sending and receiving system support DNSSEC+DANE. GMail doesn't support DANE.