/etc/courier/authmysqlrc: (or whereever you may have this file...)

MYSQL_SERVER            localhost
MYSQL_USERNAME          courier
MYSQL_PASSWORD          password

##NAME: MYSQL_SOCKET:0
MYSQL_SOCKET            /var/run/mysqld/mysqld.sock

##NAME: MYSQL_OPT:0
MYSQL_OPT               0

##NAME: MYSQL_DATABASE:0
MYSQL_DATABASE          email

##NAME: MYSQL_USER_TABLE:0
MYSQL_USER_TABLE        boxauth

##NAME: MYSQL_CRYPT_PWFIELD:0
MYSQL_CRYPT_PWFIELD     boxpwd

##NAME: MYSQL_CLEAR_PWFIELD:0
#MYSQL_CLEAR_PWFIELD    boxpwd

##NAME: MYSQL_UID_FIELD:0
MYSQL_UID_FIELD         '42'

##NAME: MYSQL_GID_FIELD:0
MYSQL_GID_FIELD         '12'

##NAME: MYSQL_LOGIN_FIELD:0
MYSQL_LOGIN_FIELD       boxname

##NAME: MYSQL_HOME_FIELD:0
MYSQL_HOME_FIELD        '/var/spool/virtual_mail/'

##NAME: MYSQL_MAILDIR_FIELD:0
MYSQL_MAILDIR_FIELD     boxname
==================================================
- and remember to enable SQL auth in authdaemonrc:
authmodulelist="authmysql"