- Fastmail can immediately cancel your account for any reason: "The Service Provider may terminate your access to any part or all of the Service and any related service(s) at any time, with or without cause, with or without notice, effective immediately, for any reason whatsoever, with or without providing any refund of any payments."

- Fastmail can disclose your info/data if it thinks it's in the interest of the company: "The Service Provider will not monitor, edit, or disclose any personal information about you [...] unless required or allowed by law, or where the Service Provider has a good faith belief that such action is necessary to: [...] (2) protect and defend the rights or property of the Service Provider; [...] (4) act to protect the interests of its members or others [...]

By comparison, mailbox.org TOS are much better.

Also mailbox.org offers GPG encryption, which Fastmail doesn't (AFAIK).