In a Mailman + ISPConfig type of setup, you may get the following error while sending an e-mail to the mailing list:

Error:

550 5.1.1 <leden@zweefvliegen-hasselt.be>: Recipient address rejected: User unknown in virtual mailbox table

When looking at the virtual aliases (in /var/lib/mailman/data/), the lists are empty.

You can resolve this by regenerating the aliases:

sudo /var/lib/mailman/bin/genaliases
sudo postmap /var/lib/mailman/data/transport-mailman
sudo postmap /var/lib/mailman/data/virtual-mailman

Afterwards, restart Postfix:

sudo service postfix restart