In AIX, in order to add the ‘Importance: High’ tag, it appears that you have to preformat your message and send it through sendmail. This can be done with a special message tag file:

contents of urgent_head:

Subject: test
MIME-Version: 1.0
Importance: High
From: name@domain.com

cat urgent_head /etc/motd | /usr/sbin/sendmail user@domain.com

This works in AIX and two of the linuxes that I tested which alias to mta. I am still trying to figure out the one that aliases to exm.