Having an HTML email does not hurt your deliverability as long as you have two things − a properly coded HTML email and a plain-text version.
The results confirmed what we assumed: HTML emails decreased open rates. What was interesting, however, was that not only were HTML emails receiving lower open rates than their plain-text counterparts, the more HTML-rich an email was, the lower its open rate.
If there are some broken tags in your HTML, the email provider and users can mark it as spam. That'll hurt deliverability − not just for that email, but also for any other emails coming from that particular email address in the future.
The emails with fewer HTML elements won with statistical significance.
Most email marketing tools will let you easily create plain-text versions within their email editor, so take those five extra minutes to create and optimize the plain-text version of your email. Otherwise, email providers such as Gmail or Outlook might think your email is dodgy. After experiencing different tests; it is found that the Plain Text version has got as much as 25% more open rates when compared to the HTML. Not only the open rates, it has also got 21% more clicks through rates as compared to the HTML emails.