Peerawit Wannawittayapa. (1999) Trusted electronic mail system on internet. Mahidol University,:ม.ป.ท.Peerawit Wannawittayapa. 1999. Trusted electronic mail system on internet. ม.ป.ท.:Mahidol University,;Peerawit Wannawittayapa. Trusted electronic mail system on internet. ม.ป.ท.:Mahidol University, 1999. Print.