Feature Request: Functionality to tell us if the customer has opened/read our response

Sometimes our users have issues that we're working on resolving with them and at a point they just stop responding. They'll then chase us even though we have replied to their last email. It strikes us as though the user isn't receiving our emails. It would be great to see if they have opened/read the emails to understand if maybe we're heading into their spam folder or if they've given up getting to a resolution (also understanding if we're being ignored for those who never respond to us in the first place). Having this available under the events tab would fit well!

I've raised this with our Commercial Account Executive and he has said we're the second customer of Zendesk to raise this feature request to him this week!