I have exchange 2007 and currently have 2 hubs. I checked the user's send items but it doesn't show up on the message tracking on exchange console. I checked the queue and its not there. This is sending to external address. It seems like sometimes it sends it and sometimes it drops. can someone point me to the right direction to debug this issue. It doesn't occur often but its annoying since we don't know where it is. Thanks

Is Outlook in cache mode? Do you see the sent item if you check via OWA?

yes i do see it on the owa and it is cache mode.

I would start with creating a new Outlook profile or set this user to online mode and see if the problem continues.

The thing is that I see it being sent out on OWA, But Message tracking doesn't see it. It seems werid to me. The same message was sent twice but only one is received. Its all internal mail also.

Correct, all sent messages will show up in message tracking - if they have really been sent. I have never seen a case personally where that wasnt true. That's why you may want to focus on the cache mode causing issues and the message itself is not actually sent. Having said that, Exchange does has duplicate message detection and will sometimes drop the second message that is resent from the sent items. However, in those cases, that will show up in message tracking as well.