Here’s an executive summary of the way Apple handles multi-part messages in Snow Leopard’s Mail.app.
What does that actually mean? Say you compose a message. The image on the right show what it looks like when you send it. The image on the left shows what it looks like to the recipient. You’ll have to click on the thumbnails to get the full versions, but you can see what I’m talking about: the Outlook user gets no cheese, no lolcat, and no text below the picture—at least not without clicking on them.
Now, perhaps I’m being too harsh by saying that Apple’s doing it "wrong". I mean, can’t we all just get along?
In this case, no, Apple is doing it wrong. One of the major features in Snow Leopard Mail was supposed to be Exchange compatibility. If you produce messages that Exchange clients can’t read, well, that’s not very compatible, now is it? There are tons of complaints about this on Apple’s discussion forums, mostly centering around Mail’s inability to read voice mail messages from Vonage– so it’s not just Exchange users who are being bitten by this.
For another day: how I used the ever-useful pickup directory to figure out exactly what the problem was.