I don't find Fastmail to be a super reliable provider, their docs aren't thorough, device support is minimal, and their customer support is a bit lacking. Spam

I don't find Fastmail to be a super reliable provider, their docs aren't thorough, device support is minimal, and their customer support is a bit lacking. Spam filtering seems iffy. Also bizarre that their mobile client doesn't cache mail. I would treat them as any other mail provider; use IMAP and SMTP with your own client, only use the features built into the protocol specs, use a custom domain. By not leaning heavily on custom features you can change providers quickly, and there's fewer things to break. Also, you should probably mirror all your mail to a different provider so you have a backup in an emergency.