Awesome. I've been looking for something like this. I know of mut and pine, but I've been looking for something new and shiny.
Edit: also fast (!) full-text search thanks to Xapian, tags instead of folders
Too bad, I was really excited about this.
folder-hook . "exec collapse-all"
set sort=threads
set sort_aux=last-date-received
# Collapse all threads by default, except if
# there's new mail in one.
unset collapse_unread
folder-hook . "exec collapse-all"
# Toggle collapsed threads with za, like in vim.
bind index za collapse-thread