On Sunday, 3 March 2013, Srdjan Todorovic wrote:

I do remember some story many years ago about an email read tracking
service that would have a hidden iframe with an 1x1 px image at a
unique URL that would load and let a paying customer see if it was
loaded. Not sure what happened to that service.

MailChimp.com offer this sort of technique for mailing lists; I would expect it's standard practice for those sort of companies; but MailChimp is the only one I've used.

Though I could never work out why we had the highest response from users on iOS devices (*) until I checked their FAQ; apparently the standard iOS mail client is one of the few that will automatically download images.

(*) of course, you get to see the full HTTP headers from the client if the image is fetched, so you get a full breakdown of the clients being used by those on the list.

Greg