I run a server on Hetzner that sends maybe 100k messages a week, and appears to have no problems with delivery. It doesn't even have SPF or DKIM set up. And I'm pretty sure that if messages were not getting delivered, users would complain. At least that's exactly what happened the one time there was a configuration screwup [1] that made some email providers reject mail from that server.
So I don't know if that theory explains the issue either, though it could be a component.
So I don't know if that theory explains the issue either, though it could be a component.
[1] https://www.snellman.net/blog/archive/2014-12-05-how-buying-...