Messages that couldn't be delivered immediately will be listed in the Deferred Messages section. By clicking on the message ID, a window containing message details will appear. Additionally in this window will be the option to delete or flush the message from the queue. The Pre Filter Deferred Messages list contains the messages that could not be scanned immediately.
Flushing a message consists of forcing the server to attempt to send the e-mail again. If the e-mail fails again, it will be placed back into the queue along with the error message received.
Flushing a message makes the server attempt to send it again. If the problem persists it will end up in the Deferred Messages section again.
Deleting a message from the queue removes it completely and it will never be delivered.