Wordpress / Contact Form 7 / Hook to wrap Header and Footer Templates around eMail HTML message body

template-mail.php

<?php
/**
 * Displays mail template parts header and footer
 */

add_action( 'wpcf7_before_send_mail', 'wpcf7_add_text_to_mail_body' );

function wpcf7_add_text_to_mail_body($contact_form){

    // get mail property
    $mail = $contact_form->prop( 'mail' ); // returns array

    ob_start();
    // get header temlate from themes root (one above)
    include_once( realpath(__DIR__ . '/..').'/mail-header.php');

    $header = ob_get_clean();

    $body = $mail['body'];

    ob_start();
    // get footer temlate from themes root (one above)
    include_once( realpath(__DIR__ . '/..').'/mail-footer.php');

    $footer = ob_get_clean();

    // add content back to email body
    $mail['body'] = $header.$body.$footer;

    // set mail property with changed value(s)
    $contact_form->set_properties( array( 'mail' => $mail ) );
}