I have seen some bloggers and internet marketers commenting on email marketing blogs, asking what email newsletter formats works best. Most answers I have seen is both HTML Email and plain text format should be done. Why? It is because richly formatted messages are nice to some people, but not for everybody and vice versa some does not want to receive plain text versions, because for them that was totally boring to read.
One of the many reasons that they do not want to receive HTML emails because their email software or programs are not capable of rendering this messages, most of the time it turns to be inaccessible to the recipient. If that continues, your clients will unsubscribe to your mailing list resulting a decrease in sales. But these days there are already email programs and some email marketing services provider that offers rendering of this kind of messages to plain text or vice versa. Though expect that there will be bandwidth and capacity issues when receiving this kind of fancy emails due to addition images, scripts, and etc.
So if you are in doubt, you can create an HTML version of your plain text message and if possible try to ask your subscribers what kind of email they want to receive in majority. Another option is to provide them both formats. Include an HTML link (visible in the HTML version only) and then repeat the URL again (visible in the text and HTML version). Be sure to test it first by sending it to your own free email accounts to see if there are some typo and grammar errors.