I built a custom email solution for a small business similar to the one presented in the article, with a bespoke newsletter creation/mailing list management/engagement reporting platform on top of it. It handles far more than 22K emails per hour, which is a trivial volume for even very modest modern hardware.

However, I do agree with the article that building a system like this can be highly cost-effective. At one point, my aforementioned client was spending nearly $75K per year to outsource this. They've been using the system I built for more than half a decade now, with the only fixed monthly costs being a few virtual private servers.