When one email goes to several people, they all receive the same tracking image and the same links, so an open or a click cannot be assigned to one person. That is why the HawkOpen extension in Gmail, Outlook on the web and Yahoo Mail tracks emails with a single recipient. If you add more people, it keeps your draft and asks you to send from the dashboard or to switch tracking off.
In the dashboard you can enter several addresses: HawkOpen sends every person a separate email, tracked on its own when that person consented to tracking. For larger groups, use a campaign: every plan has them, up to 50 recipients each on Free and 500 on Pro and Business.