When presented with a choice of Plain-text or HTML messages, the vast majority of users should select HTML. If you are using a template, HTML has effectively been selected for you.

Plain-text messages are readable on every mail client, from a cell phone to Gmail and are small in size. The format is ideal for very simple messages and maximum compatibility.

HTML messages are supported by popular e-mail programs such as Microsoft® Outlook®, Outlook® Express, Netscape® Messenger, Mozilla™ Mail, and Mozilla™ Thunderbird and webmail clients such as Hotmail, Gmail, Yahoo. A handful of e-mail programs may have trouble these messages. HTML e-mails provide considerable advantages in presentation, offering graphic design elements, branding, and improved usability.

In the end, it is still the user's decision as to what type of mail they receive. If you opt to send an HTML message, but John Doe has elected to receive Plain-text e-mails only, the message will be stripped of HTML before being sent to John. The inverse it not applicable because HTML cannot be "applied" automatically to text.