In the latest edition of Constant Contact's newsletter "Email Marketing Hints & Tips", Gail Goodman answers the question "I've heard that text emails have a better delivery rate than HTML emails. Is this true?".
The short answer is NO. Here's what Gail has to say about it:
There are two misconceptions about HTML emails:
1) they are more likely to get blocked, and
2) they are less preferred by readers.
These are simply not true. Text emails do not have a higher delivery rate than HTML and many people want the benefits that HTML provides (images, colors, readability, etc.) over text.
The deliverability of your emails is based primarily on the reputation of the sender (if you use a email service provider, they are the sender). What used to be the most important delivery element, the content of the email, is playing much less of a role these days.
And format (HTML or text) has little to no effect. When you use a service to send email, you can offer your subscribers a choice of text or HTML. That way they are getting the kind of email they want. And that is what's most important.