This article explains the reason why notification emails say Someone Viewed the Postwire, rather than providing the email address of the viewer.
When you use the Share button to share a Postwire via email, a special link gets generated for each recipient. If you invite 10 people, there are 10 unique links that get generated for the same Postwire, therefore, if a person clicks on one of those links, we can tell you exactly who viewed your Postwire.
When you copy and paste the shared link to a Postwire into your browser address bar, or when you use the Outlook or Gmail plugins, there is no way of tracking exactly who clicks to view your Postwire. In these situations, what you can see in the notification email is the viewer's company domain or ISP, and the location that's reported to us by using a reverse IP lookup.