It’s not impossible, been running my own email server for about 10 years and I inbox pretty much everywhere. I even emailed my work address and straight to inbox. I do have the full SPF, DKIM and DMARC stuff set up, for which I get notices from several email provides of failed spoof attempts.
Takes a while and effort to gain that reputation, but it’s doable. And OVH’s IPs don’t exactly have a great reputation either. Once you’re delisted from most spam databases / old spam reputation is expired, it’s not that bad.
Although I do agree it’s possibly one of the hardest services to self host. The software to run email servers is ancient and weird, and takes a lot to set up right. If you get it wrong you relay spam and start over, it’s rough.
Some modern workloads can take advantage of multiple computers. You can usually compile using things like distcc and spread the load across them.
If you make them into a Kubernetes cluster you can run many copies or many different things.
It’s still an unsolved problem: we still end up with single core bottlenecks to this day, before even involving other machines altogether.