We recently identified an issue with our mail servers which impacted a small number of our customers utilizing our Engagement tracking (formerly Open & Click Tracking) feature. In some cases, when sending an HTML message without any links which required click encoding, the tracking image was not embedded at the bottom of the message. As a result, no Open rates/events could be tracked on these particular messages. As the images were not present, there is unfortunately no way to retrieve this data, however the delivery of these messages was not affected.
For those impacted, this would have occurred intermittently for messages injected between 6/12/2013 and 6/17/2013. We have resolved this issue and image embedding is occurring correctly in all cases across all of our mail servers.
We assure you that steps are being taken to prevent this type of failure in the future. We deeply apologize for any inconvenience this has caused, and appreciate and value your support as our customers.