etc_postfix_sasl_smtpd.conf

pwcheck_method: auxprop
auxprop_plugin: sql
mech_list: plain login cram-md5 digest-md5
sql_engine: mysql
sql_hostnames: <SQL_HOST>
sql_user: <SQL_UNPRIVILEGED_USER>
sql_passwd: <SQL_UNPRIVILEGED_PASSWORD>
sql_database: <SQL_DB>
sql_select: select password from mail_users where username='%u@%r'