The key to alerts is them being valuable to the person/people who receive them.
Like Matt said I don't believe firing an alert every time someone simply opened an email and if the person/people who receive the alert feel the same then it is highly likely they will start to ignore alerts altogether.
It is the same for external emails eg if someone unsubscribes from your newsletter and this is all done on landing pages that confirm the request has been received/processed then there is no need to send the person an email that again confirms the request has been received/processed.
It completely depends on what you're trying to do. Like Alok said, you can still score or fire alerts/interesting moments off of other filters, but I'm not sure why you would want an alert fired every time someone simply opened an email.
Additionally, email opens are never a good metric to rely on, since they require image loads to be recorded. Plenty of people might open your email, not load the images, and you'll never know.
Yes, you could creare a trigger for "Opened email" and possibly change the score. Also, it's beneficial as we have filters like "Was Sent email", "Was Delivered email" and "Opened email" in smartlist filters.
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