I noticed once on one of the servers in the files /var/log/mail.log and /var/log/syslog the following warning:
postfix/local: warning: database /etc/aliases.db is older than source file /etc/aliases
This warning is displayed because someone changed the /etc/aliases file and did not update the /etc/aliases.db database.
To update /etc/aliases.db, run the commands:
cd /etc sudo newaliases
See also my article:
Redirecting mail for the root user