In a computer system that is network connectable along with one or more other computer systems to a network, a method for creating an electronic message, the method comprising: an act of creating a message item representing the electronic message in accordance with a message schema defining data fields, the data fields including at least one general properties field representing one or more general properties that are common to a plurality of different types of message protocols and a plurality of different types of message applications, one or more protocol specific property fields representing one or more protocol specific message properties that correspond to a specific message protocol, the specific message protocol being selected from among the plurality of different types of message protocols that have the common electronic message properties represented in the general properties field in common, and one or more application specific property fields representing one or more application specific electronic message properties that correspond to a specific message application, the specific message application being selected from among the plurality of different types of message applications that have the common electronic message properties represented in the general properties field in common; an act of assigning a primary type to the message item using a primary type field included in the at least one general property data field of the message item, the primary type indicating a message type associated with the electronic message; an act of assigning one or more protocol extensions to the message item using the one or more protocol specific property fields of the message item; and an act of assigning one or more application extensions to the message item using the one or more application specific property fields of the message item; wherein the method further comprises: an act of accessing the message item; an act of assigning a new application extension to the message item, the new application extension having one or more new specific properties that are to be associated with the message item; and an act of sending the message item in accordance with both the new application extension and the assigned one or more application extensions. an act of creating a message item representing the electronic message in accordance with a message schema defining data fields, the data fields including at least one general properties field representing one or more general properties that are common to a plurality of different types of message protocols and a plurality of different types of message applications, one or more protocol specific property fields representing one or more protocol specific message properties that correspond to a specific message protocol, the specific message protocol being selected from among the plurality of different types of message protocols that have the common electronic message properties represented in the general properties field in common, and one or more application specific property fields representing one or more application specific electronic message properties that correspond to a specific message application, the specific message application being selected from among the plurality of different types of message applications that have the common electronic message properties represented in the general properties field in common; an act of assigning a primary type to the message item using a primary type field included in the at least one general property data field of the message item, the primary type indicating a message type associated with the electronic message; an act of assigning one or more protocol extensions to the message item using the one or more protocol specific property fields of the message item; and an act of assigning one or more application extensions to the message item using the one or more application specific property fields of the message item; wherein the method further comprises: an act of accessing the message item; an act of assigning a new application extension to the message item, the new application extension having one or more new specific properties that are to be associated with the message item; and an act of sending the message item in accordance with both the new application extension and the assigned one or more application extensions. an act of accessing the message item; an act of assigning a new application extension to the message item, the new application extension having one or more new specific properties that are to be associated with the message item; and an act of sending the message item in accordance with both the new application extension and the assigned one or more application extensions.