queue.php and notifications loops

Hello,
I don't know if this it could be really a bug. I'm using phpBB 3.0.4 in a Windows Server 2003 dedicated machine.
After using phpBB 2.0.x for years with no problems, one month ago I upgraded to 3.0.3 and then to 3.0.4 using automatic update.

Two days ago one of my board users informed me that he was receiving the same e-mail notification every 10 minutes.
I immediately verified that the SMTP running on the same dedicated server was ok (registration e-mails, for example, were correctly sent and received).

So, I manually deleted the \cache\queue.php file that was containing the messages loop. The problem seems to be solved now.

After a few, \cache\queue.php recreates itself (probably because news mails were sent using my board) but after 3/4 minutes the same file was correctly deleted and the e-mails sent. The new e-mails were correctly received.

IUSR_machinename ("Internet Guest Account") had the following permission on the \cache directory: READ&EXECUTE / WRITE.

Why this problem has happened?
How could I make sure that the problem will not happen again?

Now I also added, for \cache directory, the special DELETE permission ("delete subfolders and files" and "delete") to IUSR_machinename account.

So, I tried to add the following to the 701 line of \includes\functions_messenger.php (after @unlink($this->cache_file):

$fptest = fopen('/phpbb/files/_testfile.txt', 'a');

fwrite($fptest, 'OK ');

fclose($fptest);

When there are one or more notifications to send, phpBB creates \cache\queue.php file.
After 3 or 4 minutes the queue.php file is deleted from the cache dir and an "OK" message is appended to _testfile.txt. It does not matter if IUSR_machinename account has deleting permissions.

So, how could I make sure that the "notification e-mail loop" won't happen anymore?
Thank you.