WordPress email reply back to the original post author

What you need to do is to modify the code that sends the email to the author and in it set the “reply-to” header of the email, pass it as part of the headers parameters to wp-mail.

Your code should be something like (adapted from the php mail function documentation).

$to      = '[email protected]';
$subject="new comment";
$message="hello";
$headers="From: [email protected]" . "\r\n" .
    'Reply-To: [email protected]';

wp_mail($to, $subject, $message, $headers);

Of course you will need to get the relevant email addresses from the post author and the comment author fields, and you will probably want to specify the “nice names” of all the people involved with the actual email address