This error kept creeping up on our front-end Exchange 2007 server:
Event ID: 4011
Process 6168[w3wp.exe:/LM/W3SVC/1/ROOT/EWS-1-128806932311406509]: Configuration for forest corp1.contoso.com was not found in Active Directory. Run the Add-AvailabilityAddressSpace command in the Exchange Management Shell for an Exchange Server 2003 Active Directory forest.Luckily the answer was on theTechNet site, I just ran this using the EMS...
Add-AvailabilityAddressSpace -ForestName Contoso.com -AccessMethod PublicFolder
Soon we'll be without a mixed 2003/2007 environment, I look forward to that day.