The quip core files at core/components/quip/model/quip/quipcomment.class.php at line 253, the $to variable were empty and there’s no coding to set the $to variable to the one that we set in the System Settings.
Hence I added a line of
$to = $this->xpdo->getOption('quip.emailsTo',null,$this->xpdo->getOption('emailsender'));
This is to get the value that we set in System Settings > quip > email > Email To field.
After adding the line of code, the notification email will be send to the email that has been set at the Email To field instead of to all Administrator's user's email.