Every power-user app has a shortcut for everything and a population of users who know four of them.
The bindings were never the bottleneck
We started by cataloguing bindings from a dozen mail clients and found broad agreement: archive, reply, next, previous, search. The vocabulary is basically settled.
What is not settled is how anyone learns it. The standard answer is a modal cheat sheet behind a question mark, which is a document you read once and never again.
Teach in place
So every action Parrot performs with a mouse announces its shortcut afterwards, once, in the same spot, and then stops. Archive by clicking three times and the hint retires itself.
It is a small idea and it moved keyboard adoption from about a fifth of active users to just over half within a month of shipping.
Home row or nothing
The remaining rule is that the common path never leaves the home row and never needs a modifier. If an action is frequent enough to deserve a binding, it is frequent enough to deserve a good one.
Everything else lives in the command palette, which is one key away and searchable, and which is where infrequently used features belong.
Parrot Mail