I struggled a bit with the ethics of this. My own experience with a client denying he received an invoice I mailed with the USPS certainly swayed my decision to

I struggled a bit with the ethics of this. My own experience with a client denying he received an invoice I mailed with the USPS certainly swayed my decision to implement it. It wasn't until I mailed the invoice with their "Certified Mail" option and disabled their web app just days before their big "debut" that they paid me. Had I not done that I have no doubt they would not have paid me. It's probably been around 8-10 years (at least) since I implemented that feature in my app and I've not had a user contact me about a client of theirs denying they've received an invoice since. That's only anecdotal evidence of effectiveness but it's worth noting. That said, I do think using pixel tracking in a marketing email is a slimy practice. I don't block it by default though. I still flag junk and delete spam once or twice a day.