When mail function fails, it does not always create error. For instance you can get error which was earlier created by this code http://api.nette.org/2.1.0/source-Caching.Storages.FileStorage.php.html#325.

Unable to send email: fopen(/home/honza/vec/temp/cache/_Nette.FileTemplate/_Email.RegisterEmail.latte-d7ce40e6e36469422781c16e335ac069.php)

I suggest to improve the exception message. This might be better:

Function mail() failed. Last error: $error[message]