This is a problem or can be a problem when someone uses HTML for message composition.
Copied from this link, which you may agree with based on what you included with your post.
http://www.birdhouse.org/etc/evilmail.html
+HTML in e-mail is a bad idea ...+
+Because HTML is for making web pages and plain text is for simple communications. If you're looking to create a web page or write a book, fine. But e-mail messages are not web pages or books. e-mail was designed for simple messaging. Anything else detracts, rather than adds to its core functionality. As Andy Roony said, "E-mail is simple. Like the pencil, it just works." Well, e-mail is not simple -- and it doesn't always work -- when HTML is involved.+
+Because it introduces accessibility problems. *When you write in plain text, the receiving mail client renders the text in whatever font the reader chooses. When you format email with HTML, the sender controls the formatting.* But this is a trap: You only think your message will render the same way to the viewer as it appears to the sender. In reality, the receiver can end up squinting because the font looks so tiny, or vice versa. HTML is not rendered the same way from one viewing client to the next - all guarantee of accessiblity goes out the window. This is especially problematic for visually impaired persons.+
When a message is composed with HTML, the HTML version is sent along with a plain text version. Some email clients include an option to display the plain text version only for all received messages. The iPhone's mail client doesn't include this option.
Long story short, there is nothing you can do about this except for requesting that all email be sent to you with plain text.