[reason=PAM auth error]
go after that one ..
so check /etc/pam.d/smtp
pam_mysql - SELECT returned no result.
it cannot find the user you're trying to log in with.
edit: put courier in debug and track the error logs .. more info to be found there when you do
DEBUG_LOGIN=1 in /etc/courier/authdaemonrc
then restart /etc/init.d/authdaemonrc restart
check your mail logs