I can not modify the 'Return-Path' header because is used for authentication. (The emails have attached some PDFs I want to fax so I use a paid fax web service and the 'Return-Path' header is used as account authentication)

To send the emails I use MIME-Lite (I send html email/with PDF attached) and sendmail:

 
MIME::Lite->send('sendmail', '/usr/lib/sendmail -t ');
$rez = $msg->send();
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About (-r) option: it's a good sugesstion. I already tried with (-f) option (in man pages seems the same with -r). The problem was I didn't used right. I tried

    $rez = $msg->send_by_sendmail('/usr/lib/sendmail -t -oem -r email.
+where.to.bounce@somewhere.com');
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but I should used

    $rez = $msg->send_by_sendmail('/usr/lib/sendmail -t -oem -r \'emai
+l.where.to.bounce@somewhere.com\'');
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This is working: The bounced emails are redirected ! but the 'Return-Path' is changed to 'email.where.to.bounce@somewhere.com' and I don't want this !!

Thanks!