Funny you mention it. To be honest, I am not asking about the mail indexer clients in this space (mu4e and notmuch.el). However, to address your concern, mu4e had even worse memory consumption than notmuch (of course my opinion but I tried both of them on and off for the lat few months, and I have to admit indexing and Emacs client memory consumption notmuch was much better in my experience).

Anyway, still need direct access to maildirs because these indexing systems do not delete the email out of your maildirs. I need to spruce up every once in a while.

Thanks for the recommendation though.