I don't understand how independent health professionals can avoid using self-hosted email. Think of your neighborhood psychologist: any email she sends to or receives from a patient or potential patient should be completely confidential, down to the email address of the correspondents. It's no secret that Google and Microsoft "read" the emails they send and receive and mine them for information.

And yet, a quick search of "psychologists near me" in Google Maps gives me a list of professionals most of whose email addresses have MX records pointing to Google servers.