Except that vit indicated in the original post that the presence of an MX record is not a reliable source of the domain's ability to receive email. Apparently DNS administrators are registering MX records all willy nilly.
Sure... but unless I'm misled, with UDP there simply is no way to know for sure excepting an out of band response—a visit to a tokenized URI sent in a message or a return message from the same domain/address. So no checks will be guaranteed. "Heroic" messages to circumvent misused standards / client-fail seems a mistake.
I have no idea where UDP came into the conversation since SMTP is a TCP protocol. I agree that there is no way to be sure unless you can deliver mail to a recipient and get a DSN or, preferred, a response. Seeing if the server is listening on port 25 and is responding in a manner consistent with a SMTP server and not giving a 521 status code is a pretty safe bet though.