New mail is not visible in Inbox

Hello All,
I have what could be a major problem here, but hopefully easy to fix. I'm migrating all email from a subsidiary office's Exchange 5.5 Server to our Exchange 2000 Enterprise Server. They are on a separate domain from us, so they have to set up an Outlook Corporate/Workgroup profile, then type in their domain username and password to authenticate to our domain controllers. They connect just fine, but when they're looking at their Inbox, new mails coming in are not visible. I have verified that I can open their Inboxes from my machine and see the mail coming in real time. If they switch their view to a different folder, then go back to the Inbox, the new mail is visible. It's as if the Inbox view isn't refreshing.
Unfortunately this didn't come up in testing until today!
Does anyone have any experience with this? Help would be greatly appreciated.
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Exchange_AdminConnect With a Mentor Commented:
New mail updates are done via UDP packets.
Is there a switch or firewall between the domains that could be blocking them?
What outlook version are you using?  turn off any cacheing that takes place.  
jbaker151Author Commented:
Yes Exchange Admin, that's what the problem was. I found that in a Outlook User Forum last night a bit before you responded. The two routers in between the new office and the mail server only had tcp opened up in the Access Lists. As soon as I added a line for udp, the problem went away.
Thanks for your response...
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