I like privacy, but a service that's focused on maximum possible privacy for its users paints a target on its back for any three-letter agency, as it will attract a large contingent of unsavoury people.

I just onboarded and was dumbfounded that they do not allow for proper calendar exposure other than a fully public link! The claim of zero knowledge is super cute, for those that need it, but I need a provider which allows me to integrate the calendar elsewhere, as those will not magically move into Proton. I guess I am not in their target market.

Seconding this - reasonable pricing and I haven't had any issues at all with the service. I haven't used FastMail but most things I read suggest they are very similar in terms of what they offer so I would think Mailbox is a good EU alternative for someone who likes FastMail. (There are also other EU providers like Tuta but with slightly different trade-offs, ie, more emphasis on privacy but at the expense of IMAP/SMTP support.)

I switched to mailbox recently and I'm finding it quite good. I set it up with a custom domain, and that did require a bit of fuzting around, but the friction there was almost all on the side of my VPS hosting service, not Mailbox's fault.

Where abouts are you located?