Adds a link at the end of all emails with a link to unsubscribe page

ltu.php

<?php

add_filter( 'wp_mail', 'ltu_add_unsubscribe_links' );
function ltu_add_unsubscribe_links( $args ) {
	if ( ! class_exists( 'IW_LTU' ) ) {
		// Let Them Unsubscribe is not active
		return $args;
	}

	$email = $args['to'];
	$user = get_user_by( 'email', $email );
	if ( $user && function_exists( 'iw_ltu_user_can_unsubscribe' ) && iw_ltu_user_can_unsubscribe( $user->ID ) ) {
		// The user is allowed to delete its own profile
		$url = admin_url( 'profile.php' );
		$url = add_query_arg( 'page', 'ltu_unsubscribe', $url );

		// Add this text at the end of the email
		$args['message'] .= <<<EOD


Delete your profile here:
$url
EOD;
	}
	return $args;
}