/etc/postfix/mysql-canonical.cf

Create this file as root, and insert these lines. Brackets indicate server-specific information.

# mysql config file for canonical lookups on postfix
# comments are ok.
#

# the user name and password to log into the mysql server
hosts = <mysql ip>
user = mail
password = <the mysql user mail's password>

# the database name on the servers
dbname = mail

# the table name
table = virtual
#
select_field = alias
where_field = username
# Return the first match only
additional_conditions = and status = '1' limit 1