A few months ago I started an Etsy shop, but I couldn’t create an account using a fastmail address. I got an “unknown error.” And the help desk suggested I use a more mainstream email service. That solution worked, and then I switched it to fastmail.

But this goes to show how basic the opsec is for some companies. Similarly with Twitter, each time I create a dev account for a bot, if I use fastmail my account will get locked 3-4 times in the first 20 minutes after creating the account (and not so with gmail). I’m not sure why they think that bad actors can’t also sign up for a Gmail account, and are more likely to use fastmail.