Attempting to monitor serial output for a string to trigger function, apparently matching string is output but the function doesn't trigger
You're confusing readlines() with readline().  The former returns an array of lines, the latter a single line.  You need to drop the [0] since you're not working with an array of lines, and you need to include the line endings that the Arduino sends:

while True:
    message = ser.readline()
    print(message.decode('utf-8'))
    if message == 'MOVEMENT\r\n' :
        send_email()
        time.sleep(0.5)