The difference between HTML and plain is effectively style over substance. While people may claim they prefer flashy, image heavy HTML emails, statistics show that plain text emails formatted simply perform much better — with plain text emails representing the best performing type of email marketing of all.
Tests conducted have found that HTML-heavy emails appear to be seen as commercial by email providers, meaning that they can often be filtered out of inboxes before they ever get the chance to be seen.
Plain text emails also represent a more personal experience for readers. Consumers don’t want to feel like they’re being sold to. Therefore an email that feels personal to the reader has a much better chance of clickthrough.