I was getting this warning after following "The Perfect Server" guide for Ubuntu.
The problem is that alias_maps was undefined in either the /etc/postfix files or in the dbispconfig mysql database, so it was defaulting to "alias_maps = hash:/etc/aliases, nis:mail.aliases".
The solution for ISPConfig 3.0 running on Ubuntu 9.04 Jaunty was to put:
alias_maps = hash:/etc/aliases
at the top of the /etc/postfix/main.cf file.
Restart postfix and confirm the change by running:
$ sudo postconf alias_maps