Sitecore ExM in Depth: How Sitecore Email Experience Manager Determine If Message Was Opened

When you look at source of html email message that was delivered by Sitecore Email Experience Manager you see strange image tag that is located before closing BODY and HTML tags:

<img height="1" width="1" border="0" style="border-color: transparent;" src="http://your.host/sitecore/RegisterEmailOpened.aspx?ec_contact_id=3D0A4C096C4287C4C50A33E8BB5DB87E04&ec_message_id=3D16BE08364227=4836B57FB8CDCEB15C85" />

It is service ExM tag that is inserted to all email. This image tag has source property link to service ExM page: RegisterEmailOpened.aspx. And as parameters are transferred IDs of contact and email. When your mail agent(or browser) tries to load this image it sends request to Sitecore server and triggers event about email opening. Of course it will not work for all mail clients and web mail portals, but it is still good way to get more information about behavior of email recipients. (One of reasons could be not downloading images by default. It is frequent setting for mobile mail agents.)