This is really in poor taste. Can you imagine if the email to Bob said "hey lets meet at the pub at 6" and the email to Alice said "hey lets meet at the pub at 4".
I know this is unlikely, but sending an email that looks like each party is getting the same thing but in fact are getting different emails just is bad.
The tracking thing is not a big deal, seems like everyone does it. Just block images and you are golden.
You're entirely correct that this part of the SMTP spec does potentially allow for dubious behavior -- and that's one of the reasons I wanted to publish this article. I have a feeling this is one of the little-known secrets of email sending which we should all be aware of.
EDIT: Also, I'd like to point one that the real textual content of a message can't differ when sending through Close.io so the scenario you explained above isn't possible within our app.
I know this is unlikely, but sending an email that looks like each party is getting the same thing but in fact are getting different emails just is bad.
The tracking thing is not a big deal, seems like everyone does it. Just block images and you are golden.