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.