Email is a text-based medium. The most common mail programs will allow prettily formatted or graphical messages to be sent, and in a one-to-one dialogue email can be a good way of sharing files; but only plain text is appropriate for a discussion list like the Forum.
Please turn off HTML or "rich text" email, and don't add any "vcard" signatures either. HTML will be removed from the message before it is distributed. Any other attachment (including "vcard" signatures, or the "MS-TNEF" object sometimes added by misconfigured installations of Microsoft Outlook or Exchange) will cause the entire message to be rejected by the list server.
There is another more subtle reason why it is important actually to turn off HTML in your mail program, and not simply rely on the server to do it for you. With HTML enabled, what you see while composing your message will typically not be the same as it will appear when distributed. In particular, when replying, your program may pretend to you that your text and the quoted text are nicely distinguishable, with the quoted text in a different colour or indented, when in fact all of this formatting is missing. If you don't actually see the ">" character, then it probably won't be there when everybody else reads your message, and the result will be an amorphous mess!
If you wish to draw the attention of the Forum's subscribers to a picture, sound clip, or web page, please send only the web address (URL) where it can be found, not the entire object itself. Please provide basic information about what the URL links to, and ensure that all links are "work safe" (ie. you could reasonably expect there not to be a problem with people viewing them at work) unless there is a good reason to include a less "safe" link and you give a prominent warning that this is the case.
See also the Mail User Agents page for more specific advice on the use of particular email programs or webmail systems.