Since there are many different email programs out there besides Outlook Express, all I can suggest is to take the time to read the help file for your email software to find out where to change these settings. I might have multiple different threads going on with the same person. I like it. So please take the time to make a short descriptive subject line on all your emails. In the beginning all email was in plain text.
Some people love HTML and some people hate receiving email as HTML documents.
Keep in mind that a 25 word plain text email is around 1K in size, while a 25 word HTML email is closer to 6K in size. I won't go into what this is doing to the internet bandwidth problem or any of that. Believe it or not, some people still pay for their internet connection by the minute. So the safe thing to do is to always send your email in plain text unless you really have a reason to do otherwise. Also some email programs can't read HTML emails at all.