Earlier quoted context omitted.
Other than whistleblowers and activists fighting the dictatorships (and they can work-around this), what is the case where not being able to prove who sent the email would be a good thing?
-- Example -- Dear Ivanhoe, I regret to inform you that your HIV test came back positive. Please contact my office at your earliest convenience to arrange a follow up. Sincerely, Your doctor
"Hey Ivanhoe, your buddy from government here. That thing that we discussed, no problem I arranged everything, T says it's cool, just wire us the money and the project is yours."
In my view, if someone is going to blackmail me for some sensitive topic like being HIV positive or dox me in revenge, solution is not that I have to go public and lie that it's not real (and risk to be counter-proven it is) - but to have police put their blackmailing asses in the jail. That's the type of protection of my freedom and privacy that I hope for.
And in the end, who has ever believed people doing public denials? Once the word gets out, by the time you publish the rebuttal majority of folks will already have an opinion on it and that will stick with you for long time no matter what you say later.