Outlook 2003 not tracking Read Receipts properly

In the grand scheme of things, this problem is minor, but I can't seem to solve
it:

Read Receipts are not tracking properly for one of my bosses.
He is receiving them, and some of the people that have responded with receipts are appearing properly on the tracking tab of the sent e-mail.

This is a mixture of internal and external recipients. The receipts were received over a period of 3 days, what got tracked or didn't get tracked seems to be at random.

We use Exchange 6 (our version is 6.5.7638.1) on Windows Server 2003 R2 SP2.

My boss is on Outlook 2003.

The receive receipts are coming from an assortment of users with Outlooks XP up to 2007.

Any ideas or any clues as to where I should even start looking?

 

This may be an obvious question but are recipients on the same network ?

 

Some are and some are not. Some of the ones on the same network have been tracked, some of them haven't.