Actionmailer ignores return-path setting


i have the problem that i want to set the return-path to the envelope
of outgoing emails so bounces can be handled. I tried to set the
header as followed in my mailer class:

  def newsletter
    subject 'subject'
    reply_to ''
    from ''
    recipients ''
    body :body => 'content'
    headers 'return-path' => ''

However, in the outgoing email the return path is automatically set to

I am using Rails 2.3.3 and Postfix 2.3

can anyone point me to the correct setting to set the return-path?