You misunderstand the way this works. The only interrogation that happens between the source mail server and the destination mail server is whether SMTP or ESMTP should be used (if ESMTP is chosen, there can be some further stuff like the format of authentication (PLAIN, MD5, SHA, etc) and whether the link will be encrypted (STARTTLS).
The flow of email looks like this:
MUA -> MTA -> MTA -> MUA
The Mail User agent on the sender's side decides if it's text or html. A mail user agent is something like thunderbird, or outlook. If the user chooses HTML, then that is what the sending MTA will pass on to the receiving MTA.
If you are really desperate to strip the html, you could write a procmail recipe that strips all the tags and execute that on the body of every incoming email.