Mailing lists are used for submitting patches
Inline? That seems wrong. Or as .patch files, i.e. attachments? Then the syntax highlighting becomes a task for whatever app opens that file.
BTW Evolution has markdown support.
I honestly don't understand your complaints about the visuals. It's plain text, sometimes HTML. It's readable. It brings the info across. You can reply, personally or to the list. What do you want, banners? Google Fonts? Big friendly green buttons? A social-media-fication of your programming mailing list?
The answer to what you're asking for / complaining about boils down to "don't use mailing lists if your community requires that amount of interactiveness and coding gamification".