Yeah its a bit worrying that you can be, em, exposed to this kinda stuff!
In short you can't delete it so that it can never be found ever, sorry!!!!
Good news is i would not panic, bin it and forget it... be carefull what emails you open in future, never reply to be removed from a listing, get some sort of spam blocker, other members will give you the low down on what's best...
I don't use one, i get hardly any spam mail and never got a porno one :-(
create a mail addie that is disposable, IE you don't need and use to register on sites ..
Only use your "proper" mail addie for family , friends and sites you can trust....
Study hard and learn to program , write one that stops this rubbish and sell it to Big Billy and make a fortune!!!!
Goood Luck dude!!!!