Every message has a message class property, PR_MESSAGE_CLASS, which identifies the type, purpose, or content of the message. PR_MESSAGE_CLASS is a required property on all new messages. A message's class determines the form that is used to present the message to the user and the folder for placing incoming messages.
Message classes are case-sensitive character strings that contain ASCII characters 32 through 127 and are delimited by periods, but cannot end with a period. Each string represents a level of subclassing and there is no limit to the number of levels allowed.
For example, most messages sent and received by client applications fall into the IPM message class, a broad category that describes all interpersonal messages, or messages that are meant to be read by a human user, rather than programmatically by a computer. Message store providers more precisely describe an IPM message by creating an IPM subclass. The IPM subclass inherits the properties of the IPM message class. Subclasses of the IPM class are named by concatenating other character strings onto the IPM identifier, such as IPM.Note to describe a note message and IPM.HelpRequest to describe a help message.
To handle the display and management of IPM messages, clients can use a standard form provided by MAPI. To handle the display and management of new message classes, client application developers have two options:
Although clients should set the PR_MESSAGE_CLASS property for every outgoing message to a subclass of either IPM or IPC, the message store provider has the ultimate responsibility for setting it. Therefore, if a client sends a message without setting its message class, the message store provider sets it to the appropriate default value for the appropriate type of client. The default message class for interpersonal messaging clients is IPM; the default message class for interprocess communication clients is IPC.
Message classes have a length restriction of 255 characters. However, MAPI recommends that they not exceed 127 characters to support the message classes used in reports. Report message classes are based on the class of the original message, with two additions: a prefix and a suffix. The prefix REPORT indicates that the message is a report and the suffix indicates the type of report: DR (delivery report), NDR (nondelivery report), IPNRN (read report), or IPNNRN (nonread report). Note that these length restrictions are given in characters; on platforms that use a double-byte character set, the actual byte count might be higher.
Message store providers should return MAPI_E_INVALID_PARAMETER from their IMAPIProp::SetProps implementations when a client attempts to assign a string that exceeds the allowable limit for their message class.