I am planning to upgrade from Exchange 2010 SP2 to SP3 for the purpose of migrating to Exchange 2013 later on. Exchange is installed in a separate Exchange Resource Forest with linked mailboxes to users in several other trusted forests.
I understand that the SP3 upgrade involves a schema update to Active Directory, but I am unsure whether I should apply the schema update to these trusted forests or not.
The documentation (https://technet.microsoft.com/en-us/library/bb125224(v=exchg.141).aspx) states that the schema update should be applied to any domains that contains mail-enabled users. But are the users in the external forests mail enabled or is it only the corresponding disabled user object in the resource forest that is mail enabled?
Do I need to apply the schema update to the external trusted forests or not?
Any help is much appreciated.
