Can't set Return-Path in emails

[HouseAddict]

Whenever I go to Send Email to Users in the admin, I can't get vBulletin to include a Return-Path. I traced the code to here:

PHP Code:

//class_mail.php
class vB_Mail
{
    /**
    * Actually send the message.
    *
    * @return    boolean    True on success, false on failure
    */
    function send()
    {
        if (!$this->toemail)
        {
            return false;
        }

        @ini_set('sendmail_from', $this->fromemail);

        if (SAFEMODE AND $this->registry->options['needfromemail'])
        {
            $result =  @mail($this->toemail, $this->subject, $this->message, trim($this->headers), '-f ' . $this->fromemail);
        }
        else
        {
            $result = @mail($this->toemail, $this->subject, $this->message, trim([B]$this->headers[/B]));
        }

        $this->log_email($result);
        return $result;
    }
} 


If I print_r the $this object from function send() I get the following

    [headers] => From: "webmaster@domain.com" <webmaster@domain.com>
Return-Path: webmaster@domain.com
Message-ID: <200811072258.ba445c190546@domain.com>
MIME-Version: 1.0
Content-Type: text/html; charset="ISO-8859-1"
Content-Transfer-Encoding: 8bit
X-Priority: 3
X-Mailer: vBulletin Mail via PHP
X-EZbouncer: http://www.domain.com/admin/ezbounce.php?u=10417

A Return-Path is being included in the headers but when I get the email it looks like the following:

Delivered-To: user@domain.com
Received: by 10.142.193.3 with SMTP id q3cs243244wff;
        Fri, 7 Nov 2008 14:05:01 -0800 (PST)
Received: by 10.90.91.9 with SMTP id o9mr3560390agb.117.1226095499865;
        Fri, 07 Nov 2008 14:04:59 -0800 (PST)
Return-Path: <nobody@host.domain.com>
To: user@domain.com
Subject: test
From: "webmaster@domain.com" <webmaster@domain.com>
Message-ID: <200811072258.ba445c190546@domain.com>
MIME-Version: 1.0
Content-Type: text/html; charset="ISO-8859-1"
Content-Transfer-Encoding: 8bit

This thread about nobody being included in the headers is all I could find. It seems like nobody@host.domain.com is the default sender in sendmail but it shouldn't be used if a Return-Path is provided.