It’s delivering email, there is no way for the receiving site to state a certificate is required anyway. Traditionally it’s just a plaintext connection on port 25 and using and supporting STARTTLS is optional.
I suppose you could write up a way to specify this in DNS but it’d take decades to be implemented and you would have to deal with pushback from the snoops who I’m sure don’t mind they can peep into the email you receive.
I guess the support of TLS was added for confidentiality and integrity on untrusted networks. If you don't verify the certificate a MITM attack is possible. So what is the remaining benefit of using TLS?
I suppose you could write up a way to specify this in DNS but it’d take decades to be implemented and you would have to deal with pushback from the snoops who I’m sure don’t mind they can peep into the email you receive.