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>
--=-=-=--