There is a plugin for Apple Mail called MailTags [1], which allows the recipient (and sender) to attach the tags he or she wants to attach to an email. Other MUAs (mail user agents, i.e. mail programs) have supported tagging for decades. Just look at Thunderbird, or some of the Emacs-based mail programs. I am sure there is a way to tag mails in mutt as well. And all that works without requiring coordination between sender and recipient. These tags are not part of the body of an Email and can therefore be used in specialised searches without risking accidental matches.

[1]: https://smallcubed.com/support/mt/manual