MessageClass Property
Returns or sets a String representing the message class for the Microsoft Outlook item or Action. This property corresponds to the MAPI property PR_MESSAGE_CLASS. The MessageClass property links the item to the form on which it is based. When an item is selected, Outlook uses the message class to locate the form and expose its properties, such as Reply commands. Read-only for the FormDescription object; read/write for all other objects in the Applies To list.
expression.MessageClass
expression Required. An expression that returns one of the objects in the Applies To list.