I like the idea, but I cannot imagine normals using it en masse. It seems so natural to those familiar with software engineering because it's basically a little language embedded into email conversations. For someone unfamiliar with computer science concepts like translation, languages, compilers, etc, it would be a formidable thing to learn and use.
This is a minor, surface-level thing and completely separate from the actual functionality of your app, but: when I read 'Mailcules', I think 'molecules', not 'Hercules.'
I'm interested to see how this turns out if you pursue it. We actually applied to this past YC with a very similar concept. We got rejected, and were later convinced by a former YC founder that it was too much of a big change for most email users, so we pivoted.
A combo-app that translated that text into a visual todo list (or other mechanism) makes more sense for 90% of users. Sort of like integrations into a more robust email client. You know like Outlook, but modern.