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The connector relies on the default viaworks Locator Active Directory Authentication and Identification Plugin that gets the users' Security Identifiers (SIDs) from Active Directory.
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Any contact that has been indexed will be searchable by any user.
How to change all indexed document SIDs from Azure AD to AD
In order to change all indexed document SIDs form Azure AD to AD you have to force connector to discover all emails in next run, instead of only new or changed ones. To do it, you have to execute "update doc.repository_state set state_value = '' where repository_id = MAILBOXES_REPOSITORY_ID" query, go through AdminWizard and set AD as auth system.