Setting Importance or Priority of an Email Message for MS Exchange.
Pass on any one of the following property in the header of the message: "Importance" "Priority" "X-Priority" "X-MSMail-Priority" Possible values are as follows: Importance header value Low Normal High Priority header value Non-Urgent Normal Urgent X-Priority header value 5 4 3 2 1 X-MSMail-Priority header value Low Normal High Ref: MS Exchange Guide