The problem started a couple of months ago. First I had reports from customers, not receiving mail sent from cox.net. I was able to get that working by going to, Service Configuration > Exim Configuration Editor > and checking the "Use callouts to verify the existence of email senders". Now, I'am getting reports that customers using my server for smtp can't send to cox.net customers. Has anyone else experienced this? Cox, I might add is of no help in resolving this. Thanks in advance for any insights you might provide.