I like the #DWIM approach of UI design, but now I remember useful examples only in Emacs and Python.

I feel it's a confusing complexity in how #OpenDKIM handles its config and the entire logic of signing or verifying.

A #DKIM milter needs to verify mail that an MTA receives from elsewhere and sign mail that the MTA sends from its own system, and these are obviously different for Postfix. So OpenDKIM duplicates some MTA settings and has a nontrivial logic which mails to sign.