/**
 * Implements hook_theme().
 */
function my_module_theme($existing, $type, $theme, $path) {
  return array(
    'my_module_mail' => array(
      'variables' => array(
        'service_name' => NULL,
        'time' => NULL,
        'therapist' => NULL,
        'booking_link' => NULL,
      ),
      'template' => 'templates/my-module-mail',
    ),
  );
}

foreach ($available_openings[$tid['tid']] as $mail_opening) {
  $variables = $mail_opening;
  $body = theme('my_module_mail', $variables);
}

<?php foreach ($openings as $id => $opening): ?>
  <div class="<?php print $classes_array[$id]; ?>">
    <?php print $opening; ?>
  </div>
<?php endforeach; ?>