How to remove "site email address on behalf of" and just keep the sender email address in "from:" part of a mail from drupal 7
Well you can write a simple module, and in your module insert this function. Here is where i got the idea 

function yourmodule_mail_alter(&amp;$message)
{
    $message['headers']['Sender'] = $message['headers']['From'];
}


If you want to go with quick sketch comment, in the link to completely get rid of the headers use

&lt;?php
function mymodule_mail_alter(&amp;$message) {
  unset($message['headers']['Sender']);
  unset($message['headers']['Errors-To']);
  unset($message['headers']['Return-Path']);
}
?&gt;


There is also a module that can help out with this mail headers


  Control and modify any (almost any) mail header. Mail Headers allows
  you to: