Btw, I used to hold tumblr in a relatively high regard when it comes to multipart HTML emails. For example, the 5 blogs worth your follow mailing, despite being pointless, has an actually legible text part that looks like it wasn't automatically generated from the HTML part. Compare it for example with Twitter's DM notification where the actual text of the message turns into “” (do not adjust your receivers, these are indeed just empty quotes). But sending out a literal lorem ipsum is a whole new level.
You know, there's absolutely nothing that would prevent you from sending a HTML-only email without a plain text part, except it being obnoxious, of course. It's only considered a good practice to provide one. Well, it's actually considered a good practice to send plain text only emails. And in fact, sending two parts with completely unrelated content violates the multipart/alternate semantics.