Latest CB 1.2 is Stable one, not RC4.

Backend ajax is always on. But as an admin (frontend is same btw) you can ignore the warnings and errors.

the param is for frontend.

@sbcglobal.net
@gmail.com
@hotmail.com

do require the IP address which sends SMTP to have an MX entry in the DNS. Typically not the case for your PC localhost, and typically the case for most webservers. That's why they reply NO to spammers which try from infected PCs.
An in that case, CB says non-valid. Please read the full description of that setting in CB 1.2 STABLE for full prerequisites for the email checker to work fine. Or just disable it in frontend, and ignore it in backend.