Reject unknown local users
local_recipient_maps
This parameter specifies optional lookup tables with all users that are local
with respect to $mydestination ("what domains to receive mail
for")and $inet_interfaces ("network interfaces for receiving
mail") . If this parameter is defined, then the SMTP server will reject
mail for unknown local users.
Put tables with bare usernames such as unix:passwd.byname and alias
maps. Example: $alias_maps unix:passwd.byname.
Beware: if the Postfix SMTP server runs chrooted, you may have to
copy the passwd database into the jail. This is system dependent.