Seems worth noting that you probably can’t track everyone, so some people may have opened it but been uncounted. If (as I expect) they work through embedding images, then someone with images disabled by default wouldn’t be counted unless they specifically enabled them. (I am such a person, but didn’t get an email this year.)
The mail merge software (YAMM) that we used gives you the option to track this. And we’re evil and used it. :/
Seems worth noting that you probably can’t track everyone, so some people may have opened it but been uncounted. If (as I expect) they work through embedding images, then someone with images disabled by default wouldn’t be counted unless they specifically enabled them. (I am such a person, but didn’t get an email this year.)