mail() Return-Receipt-To not operational

I have created a form based on the Siteground phpformGenerator, and am attempting to sendmail with a 'delivery receipt'.

I understand that the required text is;
$headers .= 'Return-Receipt-To: "'.$realname.'" <'.$email.'>' . "\r\n";

However the delivery receipt is not forthcoming.

The mail sent from Outlook contains;
Return-path:
however the mail sent via the mail() function contains;
Return-path:

I am assuming that this is the reason for no delivery receipt.

Any help wopuld be appreciated.