Author here. 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 p

Author here. 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.