This is header of mail that i have received on my email ID:-email@example.com from mailchimp:
Return-Path: firstname.lastname@example.org> Received: from mail76.us2.mcsv.net (mail76.us2.mcsv.net. [18.104.22.168]) by mx.google.com with ESMTP id er6si2501863qab.62.2012.06.03.05.09.29; Sun, 03 Jun 2012 05:09:29 -0700 (PDT)
My query is how they add the Sent to: header to the Return-Path dynamically,i also want to implement the same in my mail server,i have tried address masquereading in postfix but it was not of much help may be i am misssing something. My mail server has postfix as an MTA and i have centOs 5 as an OS.