I cannot believe I was finally able to fix the issue of mails getting bounced or marked as spam by Gmail when sent via my own mail server (yeah, I am one of those fools)…

My OpenDKIM setup was not working all along, my messages never got signed – and a really bad rep by the big G mail servers, instead.

Next task: check up for my own DNS server (twice the fool).

Relevant: The top of the DNS hierarchy.