X-MSMail-Priority

Alternative to X-Priority, although X-Priority is preferred. "High" corresponds to X-Priority: 1, "Normal" is the same as X-Priority: 3, and "Low" is the same as "X-Priority: 5". Including both X-Priority and X-MSMailPriority in the same email header is OK, although it probably best they agree.

Example:

X-MSMail-Priority: High