As other are saying this just isn't true. I've run my own email for decades and I've designed and run some pretty big commercial installations. As a small pro

As other are saying this just isn't true. I've run my own email for decades and I've designed and run some pretty big commercial installations. As a small provider, you run the risk of existing in a netblock used by other people sending spam. A small co-op I ran encountered this problem once. They were operating on the cheap and while they weren't sending spam their neighbors had been. Even as a large provider at a billion dollar company, figuring out delivery issues is a huge pain and generally not worth it. There are unofficial professional postmaster meetups around the bay and these can be helpful in getting escalation contacts to fix issues, but even with entire teams of people dedicated it's a lot to handle and is usually worthwhile to outsource the work to other companies who already have these types of relationships established.