Chỉ đọc báo cáo (khởi MMSE) tin nhắn chuyển tiếp yêu cầu tin nhắn chuyển tiếp tin nhắn xác nhận MMbox lưu trữ / cập nhật yêu cầu MMbox tin nhắn lưu trữ / cập nhật xác nhận MMbox MMbox MMbox MMbox MMbox MMbox xem nội dung tải lên nội dung tin nhắn yêu cầu xác nhận xem tin nhắn yêu cầu tải lên xác nhận xóa tin nhắn xác nhận yêu cầu xóa tin nhắn từ OMA | 346 Mobile Messaging Technologies and Services MM3 Interface MMSC External Servers The MM3 interface allows the MMSC to exchange messages with external servers such as Email servers and SMS centers SMSCs . This interface is typically based on existing IPbased Email protocols . SMTP . When sending a message to an external messaging system the MMSC converts the multimedia message into an appropriate format supported by the external messaging system. For instance the exchange of a message between an MMSC and an Internet Email server can be performed by converting the multimedia message from its MM1 binary multipart representation into a text-based MIME representation for transfer over SMTP. In order to receive a message from an external messaging system the MMSC converts incoming messages to a format supported by receiving MMS clients. Several mechanisms can be used for discovering incoming messages from external messaging servers. These mechanisms include the following Forwarding of the message from the external messaging server to the MMSC. The external messaging server notifies the MMSC that a message is waiting for retrieval. In this configuration it is the responsibility of either the MMSC or the recipient MMS client to explicitly retrieve the message. The MMSC can periodically poll the external messaging server for incoming messages. Mobile users are often able to send multimedia messages to Internet users. Owing to obvious billing issues it is usually not possible for Internet users to send messages received as multimedia messages to mobile users if the applicable billing model is the one where the sender pays for message delivery. However it is sometimes permitted for an Internet user to reply once only free of charge to a multimedia message. MM4 Interface MMSC-MMSC The MM4 interface allows interactions between two MMSCs. Such interactions are required in the situation in which multimedia messages and associated reports are exchanged between users .