I think Nick had the right of it. Consider the following...

1. Sender hits send.
2. Recipient gets email notification.
3. Sender changes mind, deletes the un-read message. Message is now deleted from the system, there is nothing in the recipient's inbox, and there is nothing in the database.
4. Recipient logs in to check mail box because of email notification.
5. Recipient sees no message, contacts admin to ask why.