Auto-detect when to use HTML or text formatted e-mails?

We are currently sending out our order confirmation e-mails in html format. This is fine for most people, but a few people are running into errors since their e-mail client does not support html. Is there some way to make it so I can select which domains to send html e-mails to, and which to send text e-mails to?

Your customers are able to select their preference from within their My Account page of your site.

Thanks, this at least gives me a good reply to give them. Do you know if there's any pre-built way to have only specific domains set to html when the e-mails are first added? Also, not all of the customers have accounts since PayPal and Google Checkout are available on the site.

Zen Cart prepares the email in both formats, and if text-only is specified, sends only the text-only version. If HTML is specified, it sends BOTH the HTML and text-only versions in the same email, leaving it up to the recipient's email client to decide which to display.
If your recipients' email clients do not support HTML messages, then the text-only portion will automatically be displayed.
Further, by default, Zen Cart assumes text-only format unless the recipient has a customer account and has specified HTML as their preference.

If you're doing some sort of manual mass-loading of customer accounts to your store, then I would recommend that you NOT set them to HTML mode if your customers are the sort which are prone to being unable to use modern email formats.