Running a mailserver with correct DKIM, DMARC etc is not that difficult.

I recently made a presentation that has a full explanation of the techniques, why they exist and how they work, on Hetzner Cloud (from the original post):

https://nh2.me/recent/Running-your-own-mailserver.pdf

I find it very easy to configure with simple-nixos-mailserver (much easier than the manual setup on Ubuntu that I ran the years before):

https://gist.github.com/nh2/6814728dc3bea1508323e9bf2213c28d