How would you create something like litmus or emailonacid

Hey guys, I want to build something like litmus.com where on signup each user will receive an email id and when you send an email to this email id you can view the email on various clients like windows outlook, outlook web, gmail for android (only screen snapshots) etc but I can't figure out what technology will I need to use in order to build this or where to start at all, Can anyone help me?