When I was using mutt, one of the biggest things that I found missing was a way to apply a filter to the entire email without creating some sort of SMTP gateway

When I was using mutt, one of the biggest things that I found missing was a way to apply a filter to the entire email without creating some sort of SMTP gateway. I wanted the ability to compose my emails in Markdown, and run the text/plain version through a filter to generate a text/html version. At the time (though I'm sure this hasn't changed), you could only run various parts of the email (e.g. only the text/plain part) through filters, but not the whole email.