Messages - the domain of the Nepomuk Message Ontology (NMO) form a large part of the body of data available on a desktop. The user sends them, receives them, forwards them to other users, stores them and recalls them when they are needed. They are almost never self contained, always refering to something - to events, people, organizations, projects, activities, anything the user might be interested in. The purpose of NMO is to bring messages to the world of ontologies so that these links can be made explicit.

The concept of a message in everyday english is broad. The Thesaurus.com website lists 'message' under 36 different meanings. NMO had to limit itself to a familiar domain of electronic communication. In NMO a message is a finite sequence of bytes containing arbitrary information exchanged between a sender and at least one receiver. Notice the difference between definitions of a Message and a File - the presence of at least one sender and receiver, and the absence of the requirement about durability. Of course it doesn't exclude the possibility of a file interpreted as a message (e.g. a .eml file).

Even with these limitations, a taxonomy of messages can be defined. Subconcepts may include an Email, a message sent with an instant messaging application, an SMS, a voice message, a forum private message etc.

A message can have multiple receivers. It can be sent to multiple Contacts. A mailing list is treated as one contact, since the message itself is sent to one email address. The fact that it is forwarded to other people is not expressed in the byte sequence that comprises the message and therefore falls beyond the scope of NIE. Blog posts and forum posts are not considered Messages in the NMO sense. This statement may seem arbitrary at first. In the case of a forum post, the actual Message according to the NMO definition (and the overall notion of nie:DataObject) is the HTTP packet generated by the browser and received by the Web Server. It is usually hardly relevant for a typical user. There are other ontologies, better suited towards description of blogs and forums, most notably SIOC.

The most important information about each Message is the sender, receiver and the dates. NMO provides properties to express this information. The choice of those properties has been inspired by header fields found in emails, but they are definitely applicable to other kinds of messages too. The nmo:references property can be used to express relations in dialogues. It would link an answer with the message it refers to.