Ok, so you use courier. The problem is that you have also dovecot installed which caused a problem during update as the ispconfig installer found the dovecot install and configured the postfix authentication for dovecot and not courier.
To fix that, edit the /etc/postfix/main.cf file and change the virtual_transport line to:
virtual_transport = maildrop
and comment out the lines:
dovecot_destination_recipient_limit = 1
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
by adding a # in front of the lines. then restart postfix.