This is happened twice, I had 2 messages inform me that my mail reader wont deal with HTML, and it does all the time with all my other messages.
Example,,,,,,I just got one from Sue Chastain(Photopshop tuts) with all sorts of pictures, links, and whatever.
Here is what the message from the other sender said. (Its an order confirmation for a new medicine cabinet, from medicinecabinetshop.com)
"You are reading this message as plain text, because your mail reader does not handle HTML."
Most likely, your mail reader is reading HTML mail just fine--but the message you received from medicinecabinetshop.com is not formatted correctly.
I have seen many examples of HTML emails that are malformed--they contain incorrect HTML that does not meet HTML standards. Some email programs can figure them out anyway; some can't. It's all up to the programmer who's creating the email program how to handle malformed HTML.
Apple's Mail program is pretty strict; it tends to reject HTML that is not written correctly. So I bet that the people at medicinecabinetshop.com wrote bad HTML in their email, and so you're not seeing the HTML.
The only way to tell for sure would be for you to go to Mail, click on the mail message, and from the View menu choose Message->Raw Source. Then paste that raw source in a post here and someone can probably tell you if it is 'correct' HTML or not.