Perhaps users are aware of relative domain names and addressing. You even see this on a service like gmail's login. A user with the address example@gmail.com doesn't have to enter '@gmail.com' when logging in - just 'example'. But actually either will do. Further it's not totally clear for a user what to enter here. Is a username/id is the same thing as an email address or not.

My aunt swears blind that an email address without the name in double quotes and the domainy bit is not a correct email address. She types the lot out.