(Chief Product Officer at Fastmail here.)

> every email ended up in spam. This included emails from myself, others from my own gmail, and even replies from people I'd emailed first.

It should go without saying, but that's definitely not the common (or expected!) experience. Our support team would be very happy to look into it for you: https://www.fastmail.com/support/

Normally when people see this kind of behaviour, it's because of one of the following: * They've connected an IMAP client that has its own spam filter turned on, and it's actually this moving all the messages to Spam, not Fastmail's spam filter. * They've accidentally mis-trained their personal filter by reporting email they want as spam.

Having said that, of course we can have issues on our end too — that's why we have a real human support team with the power to escalate to the relevant engineers.