I think it's always worth mentioning neomutt[1] when mutt comes up these days. Development is fairly active [2] and a lot of patches that people have been working on over the years have been merged, like the various header compression techniques, sidebar, search and unified inbox with notmuch etc.
[1] https://neomutt.org/feature
[2] https://github.com/neomutt/neomutt