We are having trouble with our business POP3 email. Can't send or receive. Sending from Outlook gives a "554 message: return email domain invalid". Sending a message from my Gmail to my POP3 account invites the error message:"Domain name not found". I have checked all of the DNS settings and everything looks fine. I can ping my domain, get to it with the IP address, but not with the domain name. What am I missing here?