How can I change Return-Path: www-data@localhost.localdomain?

I have a huge issue with emails being blacklisted, due to the fact that their return-path is set to www-data@localhost.localdomain, so is there anyone who knows how to change that email to use my domain name?

This is one of the errors that I can see in my email headers:

*Received-Spf: neutral (google.com: 1.1.1.1 is neither permitted nor denied by best guess record for domain of www-data@localhost.localdomain) client-ip=1.1.1.1; *

Everything is setup correctly in my PHP function for sending an email, where the sender and reply emails are using the correct domain name, but not the return-path email?

$output .= ‘From: ‘.$this->headers[‘from’].’ <’.$this->headers[‘sender’].‘>’.“\r\n”; $output .= ‘Reply-To: ‘.$this->headers[‘from’].’ <’.$this->headers[‘sender’].‘>’.“\r\n”; $output .= ‘Return-Path: ‘.$this->headers[‘from’].’ <’.$this->headers[‘sender’].‘>’.“\r\n”;

I use an Ubuntu 16.04 installation with WordPress and I’ve activated the Postfix package, which is used to send email via a contact form and a simple PHP mail() function.