There's a whole world of email mess that the article didn't even touch on: ambiguity, for example by repeating headers with conflicting values.

For example, you could have, in an email:

    Content-Type: text/plain
    Content-Type: text/html

now, maybe a virus scanner will think that the email is plain text, but the recipient's email reader parses it as HTML.

Same potential confusion with Content-Transfer-Encoding.

And then there's the matter that headers are supposed to be ASCII-only, but what happens when you send header values with 8 bits set?

And then there is how email threads work. Most of the world uses "In-Reply-To" and "References"-headers, but Exchange, in its infinite wisdom, decides to ignore them and has its own proprietary headers.

Basically every aspect of email has very annoying problems, and every attempt at a version 2 needs to either ignore some of them (annoying those particularly invested into this problem), or reinvent the whole wheel, running in Second System Syndrome worries.

It's not easy...