A certified email is perfect when you need a proof and a certification that your documents have been seen and delivered. It's like a certified or registered post mail, but, of course, a lot quicker and cheaper.
Just as the certified post mail, this is valid in legal proceedings and can be used to justify that a document or article was delivered or that a delivery attempt was made. The audit trail includes all this information, like time stamps or actions made by the addressee (opening of emails or files), to guarantee the affirmative receipt of the emails or files.
You can send a certified email in a secure way to only one person (normal delivery) or to several (batch delivery). Also, you can certify that they've opened the email, that they've opened the email and seen the document or that they've opened every single document on your mail.
When the actions have been performed by the addressee, you'll get a certification with the audit trail attached, which contains all the actions along with a time stamp. You can also download this audit from your dashboard by going to "Certified emails" and click on the document you sent.