I recently sent out a campaign and in the reports it showed that one of the subscribers had opened it 17 times. Could you explain what an 'open' actually relates to please. Obviously it is when the subscriber themself opens the email, but is it also when they forward it to someone else?
We record opens any time someone loads a specific image or if they've clicked a tracking link. So they could be opening it multiple times, their mail server could be loading the image multiple times, or if they forward the email on via their own email client anyone who opens that email would be counted as an open towards the original recipients.
You can read more about open rates here.