Change outgoing mail address from root@hostname

Hello,

 How properly change postfix for sender address rewriting from root@hostname to noreply@domain.com etc? The issue is when i try to send emails users via cwp (User accounts - Email all users) gmail will refuse to accept it because Messages missing a valid
    address in From: header, 550 5.7.1

anyone?

Thank you

Really no free support at all? Only with paid support?