It would be good if there was a filter for the email message content in pta-display-directory.php, so that we can customise the email before it’s sent.
Please look at the code for the wp_mail function, as that already includes a filter hook. My plugin uses the wp_mail function to actually send the email, so that filter hook is already there, and would be redundant to add another one. https://developer.wordpress.org/reference/functions/wp_mail/