Idea Summary
Add a parameter to the APEX_MAIL.SEND packaged procedure allowing the priority to set to High Importance. This can be achieved by using the underlying UTL_SMTP package including a Header in the data written to the email e.g. 

UTL_SMTP.write_data(lv_connection, ‘X-Priority: 2 (High)’ || UTL_TCP.crlf);

Use Case
Where we have business processes that require email sending to users, we would like to highlight that certain emails are higher priority so the recipient mail client would display them before normal priority emails.