When you use FMail2, I do not collect any information.
The only backend calls done by FMail2 are for checking for a new version and JMAP calls to reflect the email status.
Your FMail2 settings are only stored locally.
Fmail2 uses Sparkle for version checking. It works by requesting information about the currently available version from the web server. The web server will not log any user data.
I have no connections with Fastmail, Hook, Sparkle or Apple.