Hey abenton
Adding bob's comments.
To my knowledge adding a collapsible menu of any kind is not a simple task, and even if was possible with CSS and Javascript, some email clients block Javascript for safety reasons, rendering the menu static. Outlook is not great with CSS either, so chances are it may not work with some of that crowd as well even if the Javascript did run.
I'd personally render them statically or if they are long and require drop downs you could refer readers to a website where CSS and Javascript will confidently run.
I would personally love emails to become more dynamic but with the mess of email clients that currently exist that future doesn't look too bright.