Ok,
I guess the user is in cache mode on the 2k7 workstation? If so, do this:
Turn off cache mode, open outlook (is the client active)
yes - turn on cache mode from within outlook, exit
open up outlook again is the client still active?
If this resolves the issue, I have seen this a couple of times due, to the hidden message is not been synchronized. If this is the case, what version of Exchange?
--wayne