Thanks for the feedback but that value is already in my 'header' ......
$headers = 'Return-path: ' . $data_rec['cust_service_email_address'] . "\n" .
'From: ' . $data_rec['cust_service_email_address'] . "\n" .
'Reply-To: ' . $data_rec['cust_service_email_address'];
... it should work but it doesn't - any ideas?
PS. I've tried it with "\n" and with "\r\n" but the resilt is the same.
Thanks for the tip re. run time settings but I don't think this can infuence the e-mail address that e-mail failures are directed to can it?