It's possible that the email app you tried uses AWS to pull mails from all your accounts and collets them on it's own cloud. So actually not the app communicates with FastMail but the app's cloud server (hosted at Amazon) does. And the server is probably not aware that you've deleted your app and keeps on checking your mail at FM.
So I guess you should better have removed the FM account inside your mail app before deleting it. Not sure how to solve this.
At least you can try installing the app again, then add your FM account and remove it again.
Cheers,
Michael