Being able to pay for _just_ email from a company that isn't also harvesting my mail contents for ad targeting and search relevance is honestly a huge relief to me. When I have to discuss something sensitive via email -- think medical care, financial deals and other business secrets, even hobbies and interests that I wouldn't want blasted across the Googleplex -- it goes to Fastmail, every time. (And yes, I know about encrypted email and tried to do everything with GPG signatures + payload encryption for years...along with re-sending 99% of those messages b/c the recipient could do literally nothing with them.)
For business use I'm also a big fan of their mailing-list-esque offering Topicbox (https://topicbox.com). It lets me opt-in particular outside trusted vendors and collaborators for ongoing discussion w/o having the bring them onto our domain, keeps a permanent archive of our discussions, and hosts public mailing lists for newsletters, customer discussions, etc. just as well.
Disclaimer: I am not a Fastmail employee, marketing partner, or affiliate, and I have always paid full price for my service. They're just damn good at email and one of the last effective defenders against the GOOG/MS duopoly's centralization of email and IMHO worthy of regular public recommendations for that. (Self-hosting is fun and fine for personal domains where deliverability is a nice-to-have, but for business purposes I need something that _works_ and am willing to pay for that.)