If you have same problems when you send your letter, you can see it in a file `exception.log`   because in   `app/code/core/Mage/Core/Model/Email/Queue.php`  it is provided (you can see   `Mage_Core_Model_Email_Queue::send()`  at the end of method). But if *exception* doesn’t appear, it doesn’t means that recipient got your letter. It means that letter was sent to the mail service (successful execution of the function `mail()` in PHP).