In Microsoft Exchange ActiveSync, some email messages are missing even though the client continues to receive messages.
When the server that's running Exchange Server responds to a Sync command for one or more new items, the client either does not receive the response or cannot process the response. The ActiveSync client sends a Ping command for the affected folder before it sends another Sync command to retrieve the items that failed in the first response. Because the Ping command was sent, the server believes that the message was already added to the device.
To work around this behavior, disable email synchronization on the Exchange ActiveSync client, and then re-enable email for the client.
This behavior is by design. The Ping command is an acknowledgement of the Sync response.