Short answer
HubSpot's KB says more open notifications than expected usually means the email was forwarded or you copied and pasted a previously sent email. Opening your own tracked email from Gmail's sent folder does not notify by default with the Chrome extension; Outlook and Office 365 users can block self-opens. To test link tracking, click from a browser and client without the extension, or the click may be suppressed.
1. Consent comes first
Tracking settings in my demo portal show HubSpot's notice that open and click tracking may need recipient consent in some places.
2. Reading the signal
- One open: may be a preview pane or image proxy.
- Many opens from one email: likely forwarded internally.
3. Related
Email tracking consent and the never-opened stat.