I guess he's seeing his inbox empty or with just a few mails.
So, what happen if he try to send a new mail from his Z10?
I had problems with 3 users from the same server, Traveler seems to be OK, but mails were not coming to the device, sending was working though. I solved it by running a fixup on the users' mail database and then I made a reset on each device. I don't know if it could be a similar case, but it won't hurt to try.