I'm just now experiencing the direct opposite with Fastmail. Used it to avoid setting up my own mail server for a ~50 user gitlab instance. Went from trial user to paying for a full year and one day later my account is now blocked. Found this message in the first rejected email: "We couldn't figure out what the email is for. Please create your issue or comment through the web interface.". Can't send mail through webmail interface. Can't access IMAP nor SMTP. Support emails from their own support doesn't go through.

So, without any means to control it myself I'm now in the mercy of fastmail support unless I set up my own postfix/dovecot in the meantime. I guess it goes both ways.