Which Email Address is Used when an Assignee Case Update Email is Sent but the Assigned to Field on the Case is Empty

Case Update email notification Option (Setup > Support > Support Preferences > Notification tab > Update section > Notify Assignee = TRUE).

If the Assignee case update notification (Above Option) is enabled and a case is unassigned the assignee case update email notification will be sent to the Main Support email address (Setup > Support > Support preferences > General tab > Case Email Setup Section > Main Support Email address).

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