Bryan Clark was kind enough to point to Mozilla bugs 141983 and 204478 in an e-mail. Honestly I had no idea that the problem had such as storied past! The two nutcases went on about it for more than 10 years, if you count the postings to newsgroups.
It looks obvious that breaking a standard-compliant implementation is not going to happen, but even after reading these long bugs I'm still unclear on why a UI element "do not corrupt my messages" cannot be added (together with "don't send HTML" — could even be the same checkbox).
Of course, nothing of this would've mattered if Evolution weren't such a steaming pile of fail. Seems like a shining case of competition between free software projects not producing a good result. What is a kernel hacker to do?