I' ve been wracking my brain and cannot find anything related to what is happening to me. I set up a sendmail server on CentOS 6.4. I changed the loopback address to the correct IP address in the sendmail.mc and rebuilt the sendmail.cf. I've added all respective names in the local-host-names files including mydomain1.com, server.mydomain1.com, alias.mydomain1.com. I've opened port 25 in the IP tables and in the Sophos UTM. I can send mail from server.mydomain.com. I can send mail to local accounts from my exchange server exchange.mydomain2.com. But if I try to send mail to a local account from any outside server, like yahoo, gmail, or Time Warner, I get a 5.5.0 Unable to relay mail. The server is behind a Sophos UTM gateway which does IPS, Firewall, Anti-Malware, VPN, Mail Proxy, etc. However, all allowances that were made for our exchange server have been made for the sendmail server.