The ajax checker is no guarantee. It can not always check for validity of an email address. This depends entirely on how the address host responds to the test or if your host is blocking the request entirely. A lot of factors come into play, but often 3rd party sites such as custom domains fail. Hotmail and yahoo for example should work fine most of the time assuming the email doesn't have really obscure characters which too could affect it. Please upgrade to CB 1.4 as your install is out of date.