When has an email been counted as opened

[Mignon]

Hello,

can somebody tell me, when an email is counted as opened. 

Is it a downloaded Pixel? A downloaded image?

Is a preview already counted as opened?

Where can i read the information?

Thanx for your help in advance!

[shearn · Accepted solution]

"HubSpot is able to track email opens by embedding a one-pixel invisible image into the message once it is sent. When your recipient views the email and the image loads, HubSpot counts this as an open."

Full Academy article here: https://knowledge.hubspot.com/articles/kcs_article/email/how-does-hubspot-calculate-email-opens