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.