Since I'm currently reading #RunYourOwnMailServer #RYOMS, the part about #DKIM reminded me about something I heard some time ago, and I managed to find it again. Concept somewhat similar to what #OTR does.
https://blog.cryptographyengineering.com/2020/11/16/ok-google-please-publish-your-dkim-secret-keys/
https://rya.nc/dkim-privates.html