Sending emails as both HTML and TXT

Does anyone know of some “out of the box” software, or code that can be used to send emails as either HTML or TXT depending on how the recipient is going to view it? I don’t want people to get 2 versions of my email…only one. I want the email to be smart enough to present HTML if someone if viewin it in Outlook, but to show them text only if they are viewing it on a PDA…