To allow postfix accept email relaying from a specific ip adress, we can edit /etc/postfix/main.cf:
mynetworks = 127.0.0.0/8
to
mynetworks = 127.0.0.0/8, a.b.c.d/e
where a.b.c.d/e is the new group of ip address, or you can simply add one ip address in it. Restart postfix and now postfix will consider a.b.c.d/e as trusted ip and allow email from this ip address relaying email using this postfix mail server.
REFERENCE
http://www.waterworld.com.hk/en/blog/how-let-postfix-allow-some-ip-relay-email
