I have a CentOS 5 machine at home, behind a NAT gateway. Among other things, I'm running mdadm in monitor mode to alert me to RAID failures. I want it to send messages to my main email address. Any messages I send from the home CentOS box to that email address are being rejected with this error: Deferred: 451 Could not complete sender verify callout My understanding is that the destination mail server is trying to connect back to the source email host via SMTP as an anti-spam check and is failing. My external IP is set up with a dynamic DNS entry, so I guess I could set up an MX record and actually receive mail on the home box, but I don't want to deal with the security issues or the spam. Any suggestions for allowing sendmail-dependent apps running on the home machine to be able to send mail? Thanks. -Pete