You would add an RFC 2822 header to the message to indicate that the
message is high-priority.  You would do this using the addHeader()
method of the MimeMessage class.

The exact header that you would add would be dependent upon which mail
client your recipient uses.  The following headers should be added to
the email in order for most mail client applications to recognize the
email as high-priority:

Priority: Urgent
Importance: high

Cheers
GRB

Its not a mime header, they are normal mail headers. But using them is
a waste of time. Many spammers use them so they have become a spam
indicator. Most spam filters throw such mails in the bin.