Try to check if the email really came from freebitco since there are so many scammers who do this act and might they use this to deceive users to steal their password, but if they are the one who send it much better to comply on what they required so that you will enjoy their games and site features.
It's legit, Quin has already addressed the matter.
Because an email we sent you bounced back as undeliverable. Check you didn't exceed your storage limit or something and then contact your email provider to ask them why they bounced it.