[CentOS] Not receiving root mail

Although I have the alias defined in /etc/aliases and /etc/postfix/aliases, I'm not receiving root mail. Following the previous thread about unreceived logwatch mail, I tested with a manual run of logwatch, and found that my ISP is rejecting the mail because it is seeing an envelope carrying my local address. My suspicion is that the mail is going out via sendmail instead of postfix-sendmail. IIRC there used to be system-switch-mail or a similarly named package. I can't find any such package - and can't remember which file it edited. Can someone please point me to that file?

Anne