HTML IS NOT ACCEPTABLE FOR EMAIL, AND IS PARTICULARLY MISCHIEVOUS WHEN DIRECTED TO A MAILING LIST.
It all boils down to whether you are interested in every recipient being able to to read what you write. If you are writing just for exercise, it really doesn't matter, does it? If you are writing to provide an answer to a question someone posed, what is the point if they cannot read it? If you have a question, you want the best possible answer, right? What do you think the odds are that will happen if the only person with the answer you need cannot, or will not, read what you wrote?
When you compose HTML email, you probably think it looks fine, and it probably does, to you. But, are your system fonts the same size as those used by others on the list? Most likely not. Many, perhaps even most, but certainly not all. Is your monitor the same size as the recipient's? How about the distance between the monitor and the viewer? Such things cause what you write to look different to the recipient than to you.
One of the features of HTML is the ability to dictate font size. This is arguably OK for web pages. Changing fonts conveys contextual meaning for headings, titles, emphasis, demphasis and other things. But the main body, the bulk of content, should always be the system default size. Whatever the default is set to has probably been set that way because it is the most comfortable size for the user to read. He sets it based upon criteria that are of no concern to you, just like whatever criteria you use to set yours are of no concern to anyone but you. After all, most email recipients, and probably you, are using personal computers.
The problem with HTML email fonts are that, more often than not, the email program is imposing font size within the email. The most commonly used email programs do that. If your default size isn't the same as the recipient's, then he isn't seeing what you saw. Even worse, if your program or system is setting the use of a non-standard size, that size is also being forced on the recipient. If it is smaller than normal, he may choose not to read what you wrote even if he could, and he certainly won't if he can't.
HTML assumes the email reading program can accomodate the HTML markup, translating it into the different sizes, positions and other formatting we expect as a part of web pages. This in turn assumes the email reading program is using a graphical interface to provide all such formatting.
When the user's email program works only in text mode, such things are not possible. Instead, all the HTML formatting information is displayed as text right along with the content of your message. When mixed together without the benefit of a graphical interface, it all turns into an incomprehensible, unintelligible mess of mumbo jumbo.
Email via the internet was created long before the web and the graphical web browsers that make the web what it is. In the early years, all there was was text. Everyone's computer screen was text only, and not everyone was using a PC either. Many used simple text terminals hooked up to mainframe computers. Some of these terminals were teletext instead of CRT screens. Many people still use terminals instead of computers to read email, and many who use computers still use screens that display only text. It doesn't matter why, they just do.
Another problem is creating digests out of the HTML-ized email. With all of the HTML tags, email looks like line noise even when read with an HTML capable reader. When that HTML-ized message is shoved in the middle of a dozen other emails, even HTML capable readers can't properly decode it.
If you want your email read, you must not use HTML formatting.
Tools -> Options -> Send -> Plain Textor
Options -> Send -> Mail Sending Format -> Plain TextIf using Netscape Communicator, Mozilla, SeaMonkey or Thunderbird try:
Edit -> Preferences -> Mail & newsgroups -> Formatting