Returned mail error 5.7.1

Lately, I have been having an issue receiving mail from and sending mail to certain email accounts.  One of these problem accounts is AOL.  When AOL users try to send a message to my COX account, they get a bounce message back saying that the cox server will not accept unauthenticated messages from aol.com.  What is odd is that I am receiving those messages.  But when I try to reply to the message the AOL recipient does not receive my message.  The AOL recipient tells me that they receive a 5.7.1 error message.  This is very frustrating since people believe that I am not receiving their mail.  Also, they are not receiving my replies back.  Is there a solution to this?