not able to send e-mail

I have an emachine(etower 633),my ISP is bellsouth & I am unable to send e-mail to my mom whos ISP is knology.(she recieves e-mail from friends who have bellsouth)When I send her an e-mail,it comes back to my e-mail adress saying, “your mail was notdelivered to zinadashund@knology.com because the e-mail address is not listed in the public name &adress book.” Why? What does this mean?