Hello @sportintern ,
Opens are tracked through a 1x1 pixel transparent image automatically included in all emails. If they're using an email program that blocks images unless explicitly allowed to show may be resulting in them opening the emails, but that opening not actually being reflected in the reporting. Similarly, if they use an app or other program that doesn't allow image downloads to register when the email is viewed, it may not result in an open track.
See also:
Email Reporting Overview
Email Open Reports
Improving Open Rates
... View more
