Cross-forest or hybrid free/busy information lookups fail in Microsoft Exchange Server. However, standard free/busy information lookups for users in the same forest work as expected.
This issue occurs in the following product versions:
Microsoft Exchange Server 2016
Microsoft Exchange Server 2013
Microsoft Exchange Server 2010 Service Pack 2 (SP2) Update Rollup 1 and later versions
Microsoft Exchange Server 2007 Service Pack 3 (SP3) Update Rollup 6 and later versions
Microsoft Exchange Online
This issue occurs if the external URL for Exchange Web Services is not configured in the target forest.
Configure the external URL for Exchange Web Services for the target forest. To do this, run the following command in Windows PowerShell for Exchange:
Set-WebServicesVirtualDirectory -identity "server_name\EWS (Default Web Site)" -ExternalURL https://mail.contoso.com/ews/Exchange.asmx
Note In this command, contoso represents the actual domain name.
The versions of Exchange Server that are listed in the "Symptoms" section use the external URL for Exchange Web Services to connect to the target forest. Because the AutoDiscover service cannot return the external URL for Exchange Web Services if Outlook Anywhere is not enabled in the target forest, the cross-forest or hybrid lookup fails.
To verify that the mailbox is set to allow connections by using Outlook Anywhere, run the following cmdlet:
Get-CASMailbox user |fl MapiBlockOutlookRpcHttp
If this cmdlet returns a value of "$False," the mailbox is set to allow connections by using Outlook Anywhere.
For more information about the Set-WebServicesVirtualDirectory cmdlet, go to the following Microsoft TechNet website: