[slrz]

Of course, as long as you're one of the parties involved in the SMTP communication.

The problem is that even though you're trivially able to detect that TLS is not in use, the vast majority of mail providers won't act on that knowledge by refusing to send mail unencrypted (except maybe for some hosts explicitly whitelisted for that approach).

Why? Too many broken TLS setups, historically. Might be better now, I vaguely remember some push towards that from the big providers.