The registered e-mail adds two features to the standard e-mail: security and read receipt.
To send a registered e-mail, write your e-mail as usual. You can add a password to it.
When you post the message, this one is stored on the local system. Only an e-mail notification is sent to the recipients, with a URL to access the message. When the recipients receive the e-mail notification, they only have to click on the given link to visualize your message.
If you protected the registered e-mail with a password, it will be requested before your message is displayed. You will have to communicate this password to the recipients, for instance by phone.
Registered e-mails are stored in the "Register" message folder. Messages are automatically updated with the date and time when the recipients read them. Deleting a message from this folder will make it unreachable to your correspondents.