Here’s a little rant I’ve been meaning to get out for a while.
Whenever the subject of email client support of some particular email feature comes up, someone always posts a comment to the tune of “HTML doesn’t belong in email anyway, all email should be plain text”. What they’re saying is that they outright reject two important features: MIME and progressive enhancement. Given that MIME is what makes attachments (or any arbitrary binary data, attached or not) possible, I guess they can live without them too. It’s the same technology that allows web servers to identify content types, so while we’re painting with his particularly tarry brush, I guess we should remove CSS, javascript and images from HTML pages as well. That should keep them happy. With all those removed, we can all retreat to the comfort of the command line where our needs will be served admirably by the likes of the wonderful (no joke) elinks.
The whole point of the multipart/alternative data type is progressive enhancement. A client is free to select from the alternatives presented and render as best it can, with an option for manual selection (that is, as long as you don’t use Outlook which doesn’t believe in such things). This applies to the common text/plain > text/html combo as much as it would to text/plain > image/jpeg, or perhaps application/pdf > application/vnd.sun.xml.writer. Now if they restricted their comments to text/html only, I might have some sympathy, as that’s just shoddy behaviour on the part of the sender. However, they usually prefer to throw out the baby with the bath water.
To conclude: MIME is a wonderful thing; some people use it badly; get over it.