SharePoint People Picker Searching Accounts Across External Active Directory Trusts

In instances where your network administrator has created an external trust with another domain, you may want the SharePoint People Picker to find users in the external AD.

According to Microsoft's TechNet, "The People Picker automatically issues queries to all two-way trusted domains when it uses the application pool account to search for users and groups. When you select a secondary account in the People Picker, the primary account information will be returned."  Therefore, by default two-way trusts are automatically searched.

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