Think of the list as a knowledge database that needs to be browsable. Post in plain text only. No HTML, please. Outlook and Netscape users please check your default settings. Post your text inline.
Some mail programs seem to send emtpy mails with the message as an attachment.
Few people will take the extra step to open it in a text viewer, most will simply ignore it. Help keep up the friendly tone of the list. Be picky about what you write, and generous about the things you read. These don't cross borders too well. Don't worry about making spelling or grammar mistakes.