Customer since 2002. I have my backup MX pointed to Tuffmail with everything copied to Fastmail. This works for me and has guaranteed continuity twice. Once during a DNS problem at my then university about 10 years ago when I needed something time-critical from my inbox, and again during this outage.
Happy that FM is again an employee-owed company. It's responsive, ethical and good value. Also transparent. Highly recommended.