Disclaimer: I used to care enough about problems to write to support. I got canned responses to the point where I have stopped writing to them.

I am two years into Fastmail. Fastmail's web UI is meh.

I find that when I accept invites from customers, and then move the event from eg. my Billable calendar to Sales, the event is listed as "No response" -- when I respond again, the customer is sent another accept.

Showing other people's calenders gets confusing due to the default choice of colours.

The iOS app does not work offline.

The default SPF records are much too relaxed. I could understand if they just didn't enable SPF by default but allowed you to enable sane records.

You only need to scratch at the surface to find problems: https://internet.nl/site/fastmail.com

I could go on.