I'd be interested to know if there's any evidence that the Big Few mail providers, GMail or Outlook for example, penalise in the spam calculation emails coming from outside their ecosystem. I get a lot of report from people saying that emails from me are ending up in their spam bins, but I can find no reason for it, so the rumour is very seductive.

If so, having lots of people set up a "Mail Server in a Box" would be, in the longer run, a really good thing.