Here’s a simple ‘multipart/alternative’:
<#multipart type=alternative> This is a plain text part. <#part type=text/enriched> <center>This is a centered enriched part</center> <#/multipart>
After running this through mml-generate-mime, we get this:
Content-Type: multipart/alternative; boundary="=-=-=" --=-=-= This is a plain text part. --=-=-= Content-Type: text/enriched <center>This is a centered enriched part</center> --=-=-=--