In a simple, one Forest Exchange deployment, Autodiscover will work for internal users without needing to be configured. In many cases, for example we have multiple sites or multiple Client Access Servers, Autodiscover can be enhanced with a little configuration, but it will still work.
However, if you are running in an Environment that has multiple forests and using linked mailboxes, you will need to configure Autodiscover for those users who have linked mailboxes before it will work for them.
The issue is caused by the Service Connection Point (SCP), or rather the lack of SCPs. An SCP is a known location in Active Directory that services can use to locate information. As each Client Access Server is added to your organisation, it adds it’s information to the SCP. When Outlook clients start, they query their Active Directory forest SCP to retrieve a list of Client Access Servers than can be used for Autodiscover. In the case of linked-mailboxes the user’s forest contains no Exchange servers, therefore there are no SCPs to be found.
To remedy this, the SCPs from the forest containing the Exchange servers should be exported and then imported into the forest containing the linked-mailbox user accounts. This can be accomplished with the following command in the Exchange Management Shell:
This cmdlet has one mandatory parameter -TargetForestDomainController and this should be the FQDN of a domain controller in the forest that the user accounts are contained in. Other parameters can be used to specify which Domain controller to copy the information from as well as credentials.