All email tracking systems (like Vision and Absolute Automation) generate "Email Open" alerts by embedding an image file, often a client logo one pixel GIF, in the email. When this logo downloads on the prospect's computer, IS.com generates an "Email Open" alert.
Thus, if you don't get "Email Open" alerts don't assume recipients are not reading your emails. It may mean their email client does not download images automatically. This is why we encourage Vision / AA users to include email links and attachments in their emails. The best way to identify a prospect that is engaged with your company is a person who takes the next step and clicks on a link (e.g., to a whitepaper or landing page) or attachment to get more info.