KnownBooleanMetadataFieldsFAIMessage Field

Outlook email object associated metadata field. True if message is an FAI (folder associated information) message. A FAI message object is typically hidden from view and is used to store a variety of settings and auxiliary data, including forms, views, calendar options, favorites, and category lists (MAPI property PidTagMessageFlags 0x00000040 bit).

[KnownMetadataAttribute(IsEmailType = true)]
public const string FAIMessage = "FAIMessage"