Some days back, I came across an issue. No mails from Magento are sent successfully and Magento is not throwing any error message in the front end. First I thought its something related to the server and checked if the sendmail is installed properly. Then tested with a simple php sendmail function which successfully delivered the email. So I went into the log and found the following error was logged in the exception.log file.
exception 'Exception' with message 'This letter cannot be sent.' in /app/code/core/Mage/Core/Model/Email/Template.php:398
The issue was rather simple. Someone has disabled the email communication for testing purpose from the backed. And emails were sent when the option is enabled 
Cheers!
Amjath