I've started tooling around with a maildir worker a bit. Before I go too deep, I just wanted to check whether work has already been done on this, or if you had any particular design thoughts?
Side note, mbox is really not supposed to be used for a mail backend. You'll need to read the whole archive on startup, and all write operations will require you to rewrite the whole file. Also, things like flags are not supported.
I understand, but I still think it's a valuable feature to have. Not every backend needs to support every feature, and aerc can provide a nice comfy interface for reading and writing mbox files for those who are looking specifically for that.