Email received but not delivered in Exchange 2010

Hi, We are fighting for some emails that were received by the server but not delivered to user's mailbox (in Outlook or webmail).

When I use the Microsoft Exchange Troubleshooting Assistant to trace these messages, I found there is on one log entry for them, that is EventID RECEIVE. 

For other emails that are normal, I can find 2 log entries per email. One is EventID Receive, followed another one is EventID DELIVER.

So why do those emails only have one log entry? How can I retrieve them?

CliffZ