[CentOS] masquerade from - postfix

Hello guys,

recently my mail provider started blocking mails from non-existent domain.

We use for internal purposes domain labs.hlacik.eu which exists only
in our local DNS servers. Our mailserver is sx1.labs.polarion.com . As
we use virtual domains configuration & mailman  i want to keep it that
way == unchanged myorigin or mydomain.

What i want to setup is just masquerading for mails sent from localhost.

As example mail from root at sx1.labs.polarion.com should be masqueraded
to mail from root at polarion.com and etc.
This is very easy fix in sendmail, but how to do this in postfix - and
not to break virtual domains configuration.

Thanks in advance!

David