Connect postfix to postgres
cf. https://wiki.gentoo.org/wiki/Complete_Virtual_Mail_Server/Postfix_to_Database#Postgres 
/etc/postfix/main.cf
#
# Settings required to support virtual mail delivery using lookups in
# the Postgres database.
#
# A list of all virtual domains serviced by this instance of postfix.
virtual_mailbox_domains = pgsql:/etc/postfix/pgsql/virtual_mailbox_domains.cf
# Look up the mailbox location based on the email address received.
virtual_mailbox_maps = pgsql:/etc/postfix/pgsql/virtual_mailbox_maps.cf
# Any aliases that are supported by this system
virtual_alias_maps = pgsql:/etc/postfix/pgsql/virtual_alias_maps.cf
/etc/postfix/pgsql/virtual_alias_maps.cfvirtual_alias_maps
# virtual_alias_maps.cf
user = postfix
password = $password
dbname = postfix
# hosts = localhost
query = SELECT goto FROM alias WHERE address='%s' AND active='1';
Run the query on the database to verify its output:
# psql -U postfix postfix
postfix=>SELECT goto FROM alias WHERE address='testuser@example.com' AND active='1';
goto
------------------
testuser@example.com
(1 row)
Make Postfix use this database mapping:
# postconf -e virtual_alias_maps=pgsql:/etc/postfix/pgsql/virtual_alias_maps.cf
Test if the mapping file works as expected:
# postmap -q jack@example.org pgsql:/etc/postfix/pgsql/virtual_alias_maps.cf