In the last days I have reinstalled a mail server on a new virtual host and upgraded from debian etch to Lenny. The initial set was done by a friend of mine following this instructions. With the upgrade to Lenny however a few adjustments were required. So I describe the new setup here.
If you are upgrading and getting permission errors then you can fix these by berforming the steps above where permissions for /var/spool/postfix/var/run/saslauthd are set and the user group sasl is updated.