I've set up basic vim keybindings with a few caveats in neomutt, otherwise
either use the native keybindings, or consult where you got your config from:

https://seniormars.github.io/posts/neomutt/#initial-mutt-configuration

Shift + B Toggle Tree
Ctrl + J Go UP Tree
Ctrl + K Go DOWN Tree
Ctrl + E Open Up Selected Mail Folder in Tree

Standard Vim keybindings:
gg Go to top of email listing
Shift + G go to bottom of email listing
Ctrl + D next page
Ctrl + U previous page

ENTER Open selected email
Ctrl + B use mailcap to open up html/urls (use if you can't see anything)
While in this mailcap menu, you can navigate links and open it up in your
browser.

You can select certain elements and also use 'm' to open them up as well.