OK, so this is what's going on - the top-level MIME entity is "multipart/mixed", which (according to the specification) is "intended for use when the body parts are independent and intended to be displayed serially". This means that the two "text/plain" body parts are not actually two alternative views (they would have to be inside "multipart/alternative" for that), but rather two independent bodies.
The problem is that this does not actually fit into the MailMessage API. Still, we had to put the part somewhere, and we thought that the least bad option would be to put it into the Attachments collection. Technically, that's incorrect, but if we instead put it into AlternateViews, that would be incorrect as well (because the two bodies are not actually alternative body parts, as explained above).
But if all the possible options are wrong, we can at least make it possible for the user to specify which kind of behavior to use. This would be similar to the already-existing MailMessage.Settings.TreatMixedInlineAsAttachment option.
Now, we can add an option that would work like this: If there is a sub-entity inside multipart/mixed that is text/something, does not have a file name, and does not have content disposition of "inline" or "attachment", we will add it to AlternateViews collection instead of Attachments.
Does this sound suitable?