During the Lenny installation you will be asked for the name of an user (e.g. administrator) and its password.
After iRedmail install I got something like this
# vi /etc/postfix/aliases
# newaliases ->
postalias: warning: /etc/postfix/aliases.db: duplicate entry: "root"
Changing root: administrator to administrator: root resolves the problem.
iRedMail works with or without any changes like a charme but perhaps its possible that system messages not contact the email@example.com.
This happens on a fresh installation, no changes made yet.