CanonicalName: PidTagImportance
PropertyID: $0017
DataType: PT_LONG
Area: General messaging
Description: Contains a value that indicates the message sender's opinion of the
importance of a message.
Remarks: This property and the PR_PRIORITY (PidTagPriority) property should not be
confused. Importance indicates a value to users, while priority indicates
the order or speed at which the message should be sent by the messaging
system software. Higher priority usually indicates a higher cost. Higher
importance usually is associated with a different display by the user

This property can have exactly one of the following values:

IMPORTANCE_LOW - The message has low importance.

IMPORTANCE_HIGH - The message has high importance.

IMPORTANCE_NORMAL - The message has normal importance.