$eventManager = \Bitrix\Main\EventManager::getInstance();
$eventManager->addEventHandler('catalog', '\Bitrix\Catalog\Subscribe::onAfterAdd', ['СatalogEvents', 'onAfterAddSubscribe']);
Class СatalogEvents {
function onAfterAddSubscribe(Entity\Event $event) {
$fields = $event->getParameter('fields');
$userContact = $fields['USER_CONTACT'];
$mail="test@mail.ru"; // ваша почта
$subject ="Test" ; // тема письма
mail($mail, $subject, $userContact);
}
}