Mailserver configuration

[0ooNeoo0]

Dear all

M totally new for linux as well as zentyal. I have installed zentyal 3.2 communication server for Email.
1st of all i have create user example : sales@example.com then create one more users postmaster@example.com and create aliase of sales under postmaster. But it don't works error occur that salse@example.com is already exist.

2nd i have create alias 1st under postmaster@example.com and then create users called sales@example.com then create group and make sales member of it but client side it not authenticate.

1st time when i have tested its work but when actual try at production environment it wont work.

THEN CREATE 2ND SETUP

Create all pop3 ids and configure in zentyal its working fine. Issue is when ever users called sales@example.com is sending mail to xyz@example.com its getting bounce bake.
554 5.7.1 <xyz@example.com>: Relay access denied

Actually xyz id is belongs to one of associates and they all are access mail form hosting server.

is there any way to resolve it.

THANK YOU IN ADVANCE.