Exchange 2003: Issues (including migration)

Outlook is unable to download the offline address book (OAB)
Check that the folders OAB Version 2 and OAB Version 3a exist under /o=Org/cn=addrlists/cn=oabs/cn=Default Offline Address List

If they do not exist then you can recreate them by forcing a rebuild of the offline address list: -
Exchange System Manager > Recipients > Offline Address Lists > Right click the offline address list > Rebuild

If the address list does not exist at all then recreate it according to http://blogs.msdn.com/b/dgoldman/archive/2007/04/19/outlook-oab-download-fails-with-0x80004005-and-0x8004010f.aspx.

Check that there is either a replica in the administrative group / routing group, or that public folder referrals is enabled across each relevant routing group connector

See http://technet.microsoft.com/en-us/library/aa996531(EXCHG.65).aspx for further details.

Unable to see OAB for Exchange 5.5 users (co-existence)
Check that the Exchange 5.5 OAB has been generated

Open Exchange 5.5 Administrator > Site Name > Configuration > in the right hand pane open DS Site Configuration > Offline Address Book tab > Generate All

Once it is generated it will appear in the system folders under Exchange Administrator > Folders > System Folders > OFFLINE ADDRESS BOOK > EX:/=Org/ou=Site Name. It will be called OAB Version 2.

Unable to see Free/busy for Exchange 5.5 users in Exchange 2003

Ensure there is an Exchange 2003 replica of Exchange 5.5 free busy folders. This may need to be replicated to all Exchange 2003 routing groups, or else check that the routing group connectors allow public folder referrals.