Then I set up my own bare metal server and put it into a real datacenter serverhousing. I had couple of problems at the beginning and later once a few years later - problems sending to microsoft - but all those were resolved rather quickly via their forms. And I haven't had any deliverability problems since then.

The serverhousing I used is very strict regarding spam reports. I once tried to use the server also for sending newsletters (no spam lists, just emails which explicitly subscribed). Someone reported it as spam once and serverhousing contacted me soon after that they don't tolerate any spam. I explained that it was not spam and that there was unsubscribe link but I decided to not use it for any kind of marketing again. I think specialized services are better for this.