I don't understand the issue here. When you create an account with Fastmail, you agree to their Terms of Service and Privacy Policy. The Privacy Policy clearly states that, among other things, Fastmail can share your personal information with third parties who help them manage their business and deliver services. A company running an internal survey for them fits well within "third parties who help the company manage their business".
https://www.fastmail.com/about/priva...onal-analytics