For Postfix and Courier IMAP configuration you need to find out the UID and the GID number for the postfix user on your server (you need the information here and here in our ISP in a box project). Because of rights management and permissioning both Postfix and Courier IMAP need the correct user and group id to access the virtual maibox directory. The information is not in the default User management System Preference panel as postfix is a system user and not a ‘normal’ user.

You can find this out by starting up the application Netinfo Manager which you can find in the Applications/Utilities folder. Start it up and you see the panel as shown in the image below.

The Netinfo Manager application

Click on users and then in the next panel click on postfix, in the panel below you will see all the information for that user. With this method you see in the bottom panel the UID and the GID for the postfix user. On the systems I have it was always 27 but I do not know if this is always the case.