I'm fairly certain it's the 1x1 tracking image.
There are no other images with a 1px height (or width) and in the spam report it's telling me that the reason for quarantine is because an image has a height of 1 pixel. The reason I know is because my own email was quarantined by my own spam filter.
The filter is an outside service (McAfee) and is being used by my own company and in return has blocked emails from getting to our own employees.
Now I know I can have this fixed from our end by adjusting the filter settings to allow anything from ___ server/ip etc. but that's not really the point.
I can guarantee that my company isn't the only one using this service and that means that other companies that haven't made the same adjustments to the filter will also have the email quarantined.
I have a hunch that this may start showing up more often in other filters as well, if you think about it, it's not a bad method.
Having the ability to choose an image in the email that is used as the tracking agent or being able to adjust the size of the 1x1 would be a great feature.