4.6 I'm getting an error when I send mail to my own server. It says "mail for mydomain.com loops back to itself". What do I do to fix this?

Add mydomain.com to the /etc/postfix/main.cf mydestination parameter (this is the Postfix equivalent to the w class in sendmail).

What's actually happening is that Postfix hasn't been told what domains it accepts mail for and it's letting you know that with this error.