function drupal_mail($module, $key, $to, $language, $params = array(), $from = NULL, $send = TRUE) {
$default_from = variable_get('site_mail', ini_get('sendmail_from'));

// Bundle up the variables into a structured array for altering.
$message = array(
'id' => $module .'_'. $key,
'to' => $to,
'from' => isset($from) ? $from : $default_from,
'language' => $language,
'params' => $params,
'subject' => '',
'body' => array()
);

// Build the default headers
$headers = array(
'MIME-Version' => '1.0',
'Content-Type' => 'text/plain; charset=UTF-8; format=flowed; delsp=yes',
'Content-Transfer-Encoding' => '8Bit',
'X-Mailer' => 'Drupal'
);
if ($default_from) {
// To prevent e-mail from looking like spam, the addresses in the Sender and
// Return-Path headers should have a domain authorized to use the originating
// SMTP server. Errors-To is redundant, but shouldn't hurt.
$headers['From'] = $headers['Sender'] = $headers['Return-Path'] = $headers['Errors-To'] = $default_from;
}
if ($from) {
$headers['From'] = $from;
}
$message['headers'] = $headers;

// Build the e-mail (get subject and body, allow additional headers) by
// invoking hook_mail() on this module. We cannot use module_invoke() as
// we need to have $message by reference in hook_mail().
if (function_exists($function = $module .'_mail')) {
$function($key, $message, $params);
}

// Invoke hook_mail_alter() to allow all modules to alter the resulting e-mail.
drupal_alter('mail', $message);

// Concatenate and wrap the e-mail body.
$message['body'] = is_array($message['body']) ? drupal_wrap_mail(implode("\n\n", $message['body'])) : drupal_wrap_mail($message['body']);

// Optionally send e-mail.
if ($send) {
$message['result'] = drupal_mail_send($message);

// Log errors
if (!$message['result']) {
watchdog('mail', 'Error sending e-mail (from %from to %to).', array('%from' => $message['from'], '%to' => $message['to']), WATCHDOG_ERROR);
drupal_set_message(t('Unable to send e-mail. Please contact the site administrator if the problem persists.'), 'error');
}
}

return $message;
}

drupal_mail() ничего не возвращает, просто сигнализирует отработала функция или нет. Использовал черезжопный вариант проверкой на существование переменной $_SESSION['messages']['error'] после отправки письма. Сработало, как ни странно.