We recommend that you choose text format if you are unsure whether your email client supports HTML or if you use AOL, CompuServe, MS Mail or Lotus cc:Mail to read email.
Choose HTML format only if:
You read your email with an HTML-compliant email client (such as Netscape Communicator 3.0 or higher, Microsoft Outlook 98 or higher, Microsoft Outlook Express, or Web-based e-mail systems such as Hotmail), and