If you are sending a message and you need legal proof it was received you can sent a registered letter via the post office. So, for example, if you were dismissing a patient you can demonstrate that the patient actually received the message if a slime ball attorney ever challenged you in court.
The service described in the attached article is like registered mail for e-mail. It provides proof the recipient received your message.
Legally accepted proof of email delivery. Can encrypt messages and attachments. Optional digital signature. Allows markup and return of emailed contract.