Prior to that I was using imm https://hackage.haskell.org/package/imm-0.3.0.0 Looking at it's github repo it seems to have morphed into a more general-purpose tool https://github.com/k0ral/imm
I also remember looking at https://github.com/sloonz/maildir-feed and its successor https://github.com/sloonz/ua which looks nice and modular, but I couldn't figure out how to compile Go with Nix :(