This message did not have anything in itself that would show its own code by hovering over it, but the 'View Source' tool analysis did contain this rather intriguing note:
" This is a multi-part MIME message.
If you see this text, your mail client may not be able to understand MIME
formatted messages (see RFC 2045 through 2049 for further information) "
I looked up MIME on Wikipedia, to find it is a standard set of techniques for constructing e-messages containing other than ASCII text characters. The article soon lost me but I think it's something the user would not normally know even exists. I take it the "client" is actually the receiving computer or some intermediate server, so its comprehension is as it has been programmed. Not the computer's user.