This would be difficult because the email could contain any kind of content and it’s hard to predict whether or not it would work well in the context of Discourse… an email could even contain malicious executable scripts. By default the type of HTML and CSS allowed in posts is fairly limited to avoid these layout and security issues.
Something like this might be possible with iframes, but this would require the email to be hosted on an external site. You’d also need some control over that external site because embedding the email in an iframe may be prevented by a content security policy (CSP) or X-Frame-Options header. These also exist for security reasons.
To accept any email as HTML post content you’d need some sort of email parsing tool that can determine whether or not the email’s content is safe. I don’t believe this would be a simple task.