Note that the email message example in the issue you linked has no MIME-Version header field. Thus, its interpretation is effectively outside the scope of MIME. As RFC 2045 sec. 4 says:

In the absence of a MIME-Version field, a receiving mail user agent (whether conforming to MIME requirements or not) may optionally choose to interpret the body of the message according to local conventions. Many such conventions are currently in use and it should be noted that in practice non-MIME messages can contain just about anything.

@peteroupc I'm not sure I understand what you are trying to say in your comment. If you are saying the issue is invalid because the message attached to the bug report had no MIME-Version header, that was the actual message that caused the issue and the issue occurs whether or not the message contains a MIME-Version header.

If you are saying something else, I don't understand what? It is arguably correct that messages which cause this exception to be thrown are not valid messages, but they exist in the wild and should not cause the exception.

In any case, I added a MIME-Version: header to the test message in this PR, but it makes no difference. The test passes with the fix and throws the exception without it.

Since the header field makes no difference, my previous comment is irrelevant.