The Priority property returns the current priority for the
message. Setting this property value causes the X-Priority header to
be updated with the specified value.
There is no standard for specifying message priority. The
convention is to use a number from 1-5, with 1 indicating the highest
priority, 3 as normal priority and 5 as the lowest priority. Some
mailers follow the number with a space and then text that describes
the priority level.