As a Fastmail user for both personal use, as well as for my business, the best thing I can say about them is that I haven't thought about them in...a decade?

We built a Discord integration so that new emails to our support address would ping us in a Discord channel using the JMAP API. It's only failed to work once that I can recall - and that ultimately ending up being on Discord - not Fastmail.

Just rock solid service all around with no bullshit.