Cannot Access Exchange 2010 Mailbox via Outlook in other Site from where the Mbox database exists

I have just installed Exchange 2010 for my company. My company has two datacenters, SiteA will has a CAS Array and Active MBox Dag, and SiteB has another CAS Array and MBox Dag. Both sites are internet facing.

If User A has a Mbox in the DAG in SiteA, User A can connect to the Exchange server via their Outlook 2010 client just fine.  Likewise for User B in Site B.

If User A travels to Site B, User A no longer is able to connect to his Mbox in SiteA. User A can ping the CAS Array name.  Likewside for User B traveling to SiteA can no longer access his mbox in Site B from his Outlook Client. The error message when accessing/opening Outlook is:  'Cannot open you default e-mail folders. Microsoft Exchange is not availble. Either there are network problems or the Exchange Server is down for maintenance'.  The Exchange servers are working just fine.

OWA works in both sites for either users. OWA is not affected and Exchange is Redirecting just fine to the appropriate site where a mailbox is stored.  The only problem is with the Outlook MAPI.

Any ideas on why Outlook cannot connect to the Exchange Server?
digitalhitman00Asked:
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digitalhitman00Author Commented:
There was a network issue where a 'firewall' type device was only allowing port 443 between both sites. This explains why the users could access OWA.  Once we opened up other exchange ports between both sites...BAM! Everything starting working.

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