[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
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