Email supports multiple MIME types, so if someone sends you an HTML email, they should also be converting that HTML into a text representation as well so you can choose (or your email client can choose) which version you want to read.
Converting from HTML to text isn't that difficult, there are a number of open source libraries that'll do it. The harder problem is if someone sends you an email with a lot of images or where the layout of content is important, which is why people typically use HTML, it's much harder to map those aspects to plain text.
Converting from HTML to text isn't that difficult, there are a number of open source libraries that'll do it. The harder problem is if someone sends you an email with a lot of images or where the layout of content is important, which is why people typically use HTML, it's much harder to map those aspects to plain text.