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