Set up the Email Integration in HubSpot and all your emails will track - this is available on Hubspot's Free CRM!
Watch this video to see how to connect your email inside Hubspot to track emails (log new contacts with an email address) and to track email opens.
Email integration for logging emails and creating new contacts is free forever on the HubSpot CRM. However, how many "emails opened" and "email clicked" notifications is limited on the free version. Upgrading to HubSpot Sales Starter or higher will give you unlimited email open/click notifications!