I'm not sure what would be gained by that compatibility break. If it was something like the "upgrade" from plaintext email to HTML email, where both could be s

I'm not sure what would be gained by that compatibility break. If it was something like the "upgrade" from plaintext email to HTML email, where both could be supported in the same message, I think that could be a reasonable approach, but I can't imagine anything that could be improved at that level.