A Message resource is what you might think of as a message "header" or "envelope." It is a small description of the message (from, to, time, etc.), so that loading a list of messages is efficient; the actual message content is provided in the form of links to one or more attachments.
There is no fixed limit on the number of recipients a message can have. Therefore, the recipient list is also provided as a link to an attachment; because there is no limit on the number of recipients, loading recipients into the Message resource could have a big impact on performance.
Non-delivery receipts (NDRs) have one additional child resource, FailedRecipients, which includes a Recipient URI and a failure status code. New attribute "FutureDelivery" can be set while sending voicemails at specified time in future.
Beginning with Cisco Unity Connection 10.5 and later, when one or more tenants are configured on a single installation of Cisco Unity Connection, a user with Mailbox Access Delegate Account role and belonging to a particular tenant will be able to send messages of all the users within the same tenant only.