What is the best Free Tier email tracking out there?

Greetings HN Community! I am currently in the process of putting together a very smallish monthly niche Newsletter -- for which clients will pay very smallish amounts of money to subscribe to. I am doing this in an attempt to bootstrap a very smallish business from scratch, basically. Since it would be a "bad thing" (you know, compare to Martha Stewart, "it's a good thing" -- well this would be the reverse of that! :-) <g>) if said Newsletter were to be sent but, for whatever reason, subsequently not received and/or not read, I'm looking for a way to quickly and effectively know if the Newsletter was received and read. I know there's a bunch of websites out there which do email tracking, but I'm ideally searching for the following characteristics: a) Free Tier -- ideally more than 30 days long. b) No need to use the vendor's site to send emails; i.e., just insert URL or tracking pixel image or HTML or whatever into each individual email message (maybe this is a bad idea however, after all, how would a vendor/SAAS know to start tracking something sent from Gmail? Hmm, maybe a bcc? Feel free to suggest ideas and alternatives here...). And finally: c) I'm looking for people to discuss their experiences with the Free Tiers of email tracking providers out there -- any that people will give rave reviews? Any that should be avoided? Any caveats or other things to be aware of that you learned while working with an email tracking provider? etc., etc. OK HN Community -- The floor is all yours! (And many thanks in advance -- for any and all feedback!)