Users in an Exchange Server 2013 environment cannot obtain free/busy information in a cross-forest setup when the number of AvailabilityAddressSpace objects exceeds 100. In this situation, the following error is returned:
Failed to get freebusy response for user:EmailAddress = <>SMTP:firstname.lastname@example.org, Attendee Type = Required, Exclude Conflicts = False. Message: Unable to send cross-forest request for mailbox <UserName>SMTP:email@example.com because of invalid configuration., inner exception: Microsoft.Exchange.InfoWorker.Common.Availability.AddressSpaceNotFoundException: Configuration information for forest/domain domain.com could not be found in Active Directory.
at Microsoft.Exchange.InfoWorker.Common.Availability.TargetForestConfigurationCache.FindByDomain(OrganizationId organizationId, String domainName)
at Microsoft.Exchange.InfoWorker.Common.Availability.QueryGenerator.GetTargetForestConfiguration(EmailAddress emailAddress)
This problem occurs because 100 was hard-coded as the maximum number of AvailabilityAddressSpace objects that can be read from Active Directory Domain Services (AD DS) in Exchange Server 2013.
To resolve this issue, install Cumulative Update 8 for Exchange Server 2013.
For more information about this issue, see Add-AvailabilityAddressSpace.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.