What is certified (authenticated) e-mail?

Certified or authenticated e-mail is an e-mail where the sender (usually sending commercial and/or especially important messages) pays a small fee to an authority for ensuring a safe delivery of that e-mail to the recipient's mailbox.

Certified e-mails are often used in communication between commercial parties that extensively use spam-filtering systems and a certified e-mail can easily bypass them making sure the message reaches the intended recipient.