Similarly, a sender of a message might be the News system, or the author who initiated the submission by using News. Messaging is not necessarily interpersonal. It is also quite normal for different applications to communicate by using messaging methods. A prime example of this would be EDI traffic. It is clearly messaging, but not interpersonal.
User agents are the interface through which a human user interacts with the messaging system.
On non-interpersonal messaging user agents may be built directly into applications. MTAs are used to transport messages from one computer system to another. An example of a good MTA is ZMailer. They are used for storing messages before and while UAs are used to access and manipulate them. Gateways are conceptually made of two MTAs and a UA that interconnects them.