With a registered email you receive a receipt for the delivery, similarly to a registered letter. With RMail, however, this receipt is stored digitally. In addition, this Registered Receipt contains a digitally signed and encrypted version of your original email. This allows you to authenticate the integrity and originality of the entire process at a later date.
This allows you to prove the content of your message, including the email addresses involved. RMails are emails with high evidential value when it comes to who said what, when, how and to whom.
Additionally, whenever you send an RMail message, you economise delivery costs compared to postal shipment.