When you enable Email Tracking, Cirrus Insight will embed a tiny, transparent image pixel in the email. To register an email open, that pixel has to be "seen."
When you receive a reply to your tracked email but not an open count, this generally indicates that the recipient's email client blocks images by default or that a pixel-blocking extension is being used. In these events, Cirrus Insight's Email Tracking feature will not function.
The default settings for the Outlook and Gmail email clients are:
•Microsoft Outlook is configured by default to block automatic picture downloads from the Internet.
•Gmail has image download enabled by default.
For more information on Email Tracking, please watch this short tutorial on Email Tracking.