If you need the reader to do something, then write "ACTION NEEDED", "REPLY REQUESTED", etc.
If you need the reader to schedule, then write "DUE BY X", "DEADLINE IS X", etc.
If you need the reader to be fast, then write "URGENT", "EMERGENCY", etc.
If your message includes dates or times, and can be read by people in different date-time regions, then clarify your dates and times by using a global date format "YYYY-MM-DD" and a global time format "HH:MM TZ" with 24-hour time and a time zone abbreviation (e.g. "13:00 PT" is 1 p.m. Pacific time zone).