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 strict 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.