If we go back a few (decades) years - to the time of the registered letter, the only proof of message sent was validated by a third party. The law didn't care if the message was received only that it was sent. This wasn't perfect either.

I tend to have a strict rule that mission critical mail/correspondence needs to be confirmed as received - either electronic acceptance or personally. We therefore build that request into our original mail. If I have not received reply, I phone, as it is in my interest to get the reply.

Eg. We issue printing order for JHB workshop, we ask printer to acknowledge receipt of mail and AGAIN acknowledge when job is delivered.

So far so good!