I am loving this at the moment!!....
It will let you know who opened your emails and who did not, when, how many times.COOL HUH!
I am not an affiliate or trying to plug this, I am just finding this great to use for emails, and testing email you can do this, and keep using split testing to see which subject lines and emails get opened.
Then when I have found a good headline or subject lines that are getting read, I put or add it to my funnell autoresponder. Boom!
Hope it helps.