IMAP is a flawed attempt to solve the problem. This means that IMAP is more deserving of criticism.

Yes, but it's also the only attempt to solve the problem. (Also, I'd argue that POP is a half-assed attempt to do anything, and is more deserving of a swift kick in the ass.)

Which, of course, is not to say that IMAP can't be improved. Searching multiple folders, or at least allowing a search on a non-selected folder, would make a lot of sense. I suppose annotations aren't a bad idea, but it's nothing I would be very interested in, and I doubt it's of much interest to many people. And you can insert the exact same message that already exists. The big differences are that it would have a different UID and be in a different position in the natural order of the IMAP folder. The envelope, if you're talking about an SMTP envelope, isn't recorded on an IMAP server, IIRC, and, in this case, the replacement message wouldn't even have one, as it never went through SMTP channels.

Oh, I think the BODYSTRUCTURE is pretty well defined in the BNF at the bottom of RFC2060.