External Short Messaging Entity (ESME) is a term originally coined by Aldiscon

Aldiscon

Aldiscon was a telecommunications software company founded in Dublin, Ireland in 1988. The company was an innovator in supplying software products to the mobile sector and became a market leader in the supply of Short Message Service Centre's to mobile operators globally.Aldiscon invented SMPP,...

 to describe an external application that connects to an SMSC

Short message service center

A short message service center is a network element in the mobile telephone network which delivers SMS messages.-Operation:When a user sends a text message to another user, the message gets stored in the SMSC which delivers it to the destination user when they are available...

 to engage in the sending and/or receiving of SMS

Short message service

Short Message Service is a text messaging service component of phone, web, or mobile communication systems, using standardized communications protocols that allow the exchange of short text messages between fixed line or mobile phone devices...

 messages.

SME is a term used in many cellular circles to describe a network entity (mobile/cell phone) that can send/receive messages. ESME is essentially one of these but without all the wireless aspects; i.e. it is connected via TCP/IP, X.25 or similar.

Typical examples of ESMEs are systems that send automated marketing messages to mobile users, voting systems that process SMS votes (Pop Idol

Pop Idol

Pop Idol is a British television series which debuted on ITV on 6 October 2001. The show was a talent contest to decide the best new young pop singer in the United Kingdom, based on viewer voting and participation. Two series were broadcast - one in 2001-02 and a second in 2003...

, Big Brother

Big Brother (TV series)

Big Brother is a television show in which a group of people live together in a large house, isolated from the outside world but continuously watched by television cameras. Each series lasts for around three months, and there are usually fewer than 15 participants. The housemates try to win a cash...

), etc. Basically, anytime a mobile user sends or receives a message where the other party was not another real mobile user, it is likely that the other sender/receiver is an ESME.

SMSC uses protocols such as SMPP, UCP, OIS

Object Oriented Input System

OIS is a code library for constructing a human-computer interface with input devices such as a keyboard, mouse or game controller. OIS is designed so that software developers can easily use input from these devices with a computer application.-General information:The Object-Oriented Input Library...

, CIMD

CIMD

Computer Interface to Message Distribution is a proprietary short message service centre protocol developed by Nokia for their SMSC .-Syntax:An example CIMD exchange looks like the following:...

, SMCI all of which denote the concept of an ESME connecting to an SMSC.