Syncing Maildirs with mbsync is hard and
brittle, but better than anything else
Gnus (with evil-mode bindings) is powerful
and easy to use
provides a way to feed information in to Gnus, be it RSS or twitter
feeds, or whatever. Extending it is as trivial as writing a program
which emits JSON objects and manages to
keep its state enough to not repeat object IDs.
Adaptive Scoring is
important. user-transparent machine learning and a pattern that more
systems can use. This is only implemented in Gnus but I have a sketch of
Decoupled Adaptive Scoring that
I would love to see exist some day.