In order to track whether an email has been opened, a transparent 1x1 pixel image is embedded inside each message. This is the standard method for tracking open rate that all email services use. An "open" is recorded each time that image is displayed to a recipient, so the open-email tracking relies on images working in the email client.
Note: Images can't be included in a plain-text email, so open tracking doesn't work when you use the Plain Text eNotify option. Similarly, the tracking tags added to hyperlinks for click-through tracking can't be added to URLs you put in a plain-text email.
An "open" registering in the reporting does not prove that the recipient actually read your email. For example, the tracking image may display in the recipient's reading pane just before the recipient deletes it.
Even when the recipient reads the email, an "open" will NOT be logged if:
The recipient's email reader cannot receive or display images (text-only email client).
The recipient's email reader is set to display "text only" (some people do this to improve email downloading performance).
The recipient has configured the email reader to suppress images and does not elect to display them (many email readers, such as Yahoo! and Outlook, do this by default).
The recipient reads the email only in a viewing pane that does not display images.
The recipient downloads the email and reads while offline (as images are referenced in the HTML message and displayed dynamically, reading the email offline does not allow the tracking image to be called upon to display).
The recipient clicks a "View online" link (if present) from a preview pane and reads the web page version in a web browser.
The recipient is using privacy software that blocks tracking images from displaying.
Some miscellaneous connection blip or server hiccup causes the tracking image to be unavailable at the moment it is called upon to display (this would be rare, but possible).
Generally, you will find the reported open rate to be lower than the actual open rate. Including links in a message will increase the accuracy of your estimates of how many people read your eNotice, because even though a user may block images, a click on a link within the eNotice will register as an "open."
Also worth noting is that open-rate reporting happens in real time, so the open rate will likely increase over several days following the mailing.
Unfortunately, due to the way opens are tracked and the ways many people view their emails, the opens report will never be 100% accurate. It's better used as a relative measure you might use to compare two mailings to the same list, rather than as an absolute measure.
Troubleshoot a broken tracking image
If you see a broken image link in an eNotice where you haven't inserted an image, it's most likely the tracking image that alerts eNotify that a recipient has opened the message.
eNotices have a 1x1 pixel image in them that allows you to track the number of recipients who opened your eNotice. For more information about the tracking pixel, see Track email open rates.
In instances where an email client is set to not display images, all images in the email, including the tracking image, will display as broken. Allowing the images to display should resolve that issue.
If all the other images in the eNotice display, but the tracking image still displays as a broken image, it is probably due to a privacy setting in the recipient's email client or email server that suppresses tracking images. Adding the email address to the allowed senders list can cause the image not to display as broken. Obviously, this is within the control of only the recipient, so there is not a way to ensure that tracking images are not suppressed.
Outlook 2007 and Outlook Web Access (OWA) suppress tracking images (or "web beacons") by default. The web beacon suppression can be disabled for your own email by your email administrator, if desired.