1. First Two Way Transitive Trust is configured between these forests
2. When target Forest resolves the Source domain for Free/busy it should resolve “Autodiscover.domain.com” to your source forest client access server
3. You can use a GALsync to have your Mailboxes synced from source forest as contacts in the Target forest.
4. You can use ILM, FIM 2010 or you can use 3rd party NetSec Galsync which will get the contacts synced from Source to Target.
1. Now you got to add your AvailabilityAddressSpace on the source forest
2. Run the Below Command on the Source ForestAdd-AvailabilityAddressSpace -ForestName Targetdomain.com -AccessMethod PerUserFB -UseServiceAccount $true
3. Giving "ms-exch-epi-token-serialization" permission to the Source Exchange Servers over the Target Forest
4. Run the Below Command on the Target ForestGet-ClientAccessServer | Add-AdPermission -AccessRights ExtendedRight -ExtendedRights "ms-exch-epi-token-serialization" -User "Exchangehostname\Exchange Servers"
5. Run this command in the Source forest to export the SCP from the Source forest to the Target forest, So that the Target forest will have the free busy information of the Source forest
6. Save the Credentials of the Target forest.$Target=Get-Credential
Please Type “DomainName\Username” and Password
7. Now runExport-AutodiscoverConfig -TargetForestDomainController "Targetdc.targetexchange.com" -TargetForestCredential $Target -MultipleExchangeDeployments $true
8. Now Autodiscover information has been exported from source forest to the Target forest
9. Now able to use autodiscover & Access free busy without any issues
Hope It will help you.