Multiple recipients hidden when receiving message through exchange

Hi! hope this is an easy one to solve. I have set up a few SBS servers with exchange before, but haven't come across this problem.

We have a simple domain setup with a 2003 SBS and the built-in Exchange server running. Exchange downloads email off the default mail server using the POP3 connector, then distributes them to the AD user's mailbox.

When an email is sent only to the exchange user (ie they are the only recipient in the sender's "To" box) the email they receive shows the recipint's name in the "To" field (as it should). However, when there are multiple recipients (ie 2 or more names in the "To" box of the sender), the "To" field in the exchange users mailbox is empty... So thay cannot do a reply to all or even see who elso the email was destined for. It's almost as if everyone was Bcc'd, but I know thay weren't, because I have tested it myself.

I'm assuming it is an exchange problem/setting not an Outlook one because:
a) we have the same problem on all client machines
b) we have the same problem using OWA

Here's what I have found out so far:

a) I have checked the original mail server (the one exchange pop connector connects to) and everyone's names are in the "To" field, so it is definately at the Exchange level.
b) I have exacly the same setup elsewhere, and do not have this problem, so I know it is not a limitation of the pop connector or anything.

I'm hoping it is simply a setting on the Exchange server that I am not aware of, but any help would be appreciated.

thanks in advance.