If you have same problems when you send your letter, you can see it in a file exception.logexception.log
because in app/code/core/Mage/Core/Model/Email/Queue.phpapp/code/core/Mage/Core/Model/Email/Queue.php
it is provided (you can see Mage_Core_Model_Email_Queue::send()Mage_Core_Model_Email_Queue::send()
at the end of method). But if exceptionexception 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()mail()
in phpPHP).