If an employee's email matches an existing employee already on the Custr dashboard, that record will update to reflect the Office 365 Integration. You will see this update with the Office 365 icon next to their name (see image above).
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The polling rate is set to every 24 hours, meaning the employee records will be updated and removed overnight in line with the Office 365 Directory.
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You don't have to import all employees in the list. You can untick the ones you don't require, such as shared mailboxes, resource mailboxes, and admin accounts.