Email can be lost in any number of ways from any type of email system. And when they are lost there needs to be a quick means to recover them. Each type of email loss requires a different means of email recovery.
Web based email can sometimes be recovered provided you have recently opened the message on your computer. If you have you can use a powerful general-purpose data recovery tool to scan your temporary Internet files folder. This will usually return the lost email. REMO Data recovery make several programs that can handle this scan. Recommended is REMO Recover (Windows).