No need to bother with a tracking image. Just scour the message headers of any message sent in the past--there's decent chance there's a few more dates lurking in those headers, and even an IP address or two.
Years ago they would include the IP of the end user. Gmail and hotmail stopped including that more than a decade ago. But they both used to. I think Gmail stopped this practice years ahead of Hotmail.
I have a relative with mental health problems who lives on the street, and he sometimes writes me from public libraries -- kind of luckily for family members who are concerned about him, the webmail service he uses still includes that info in headers. So I usually know which public library he writes from.
