This file is the template file for the way the email that gets sent out looks. If you want to style the email (html) this is where you would do it. If (for some reason) this had been given a dark background, then the email that is sent out would have a dark background.
But that is unlikely, if you have not done anything to it.
Three people on three different machines definitely reduces the odds of it an being Outlook Express problem, unless the Tech admin is on the way out.
Have they forwarded one of the emails to your machine, if so, how did that one look?