Opens are triggered when a recipient opens the email ( and images are automacially downloaded). More information on opens and clicks can be found here.
You may see unusually high open rates connected to one or more emails. This can be due to several reasons:
- every click into the email counts as an open
- users may be forwarding the email
- Preview pane: if a user has the preview pane enabled with Outlook, an open is triggered every time it pops up. This can happen with every click into the email, or when a new email shows up in their inbox