As Fastmail themselves have pointed out many times, e-mail is inherently a multi-party activity with both parties holding a copy of the e-mails. Aside from the microscopic amount of people GPGing their e-mail and sending it over good servers, your e-mail is as private as the other party treats it. Which isn't very much. And you can't do anything about that.
You want private and / or anonymous communications? Don't use e-mail.
You want private and / or anonymous communications? Don't use e-mail.