People need to learn to just use registered mail. Yes, it's ridiculous that it's necessary, but the postage and hassle is probably less than dealing with those intentional hurdles.

Alternatively, if the US legal system allows it and you can find a number: Fax. This has the advantage that it can be automated on your end so it's not much more hassle than a quick e-mail, and the delivery receipt (yes, trivially spoofable in theory, but I would assume it's widely accepted in practice) also shows what the content of the message was.