You should only use R.S.V.P. on an invitation (to a party or social event, wedding, etc.) It is usually expected on invitations, but to see it on a letter would be odd. In fact I would go as far as to say that to directly request a reply to a letter, however politely, would be rude. You should only suggest that you would like to get a reply, and Sloopjc's examples will do very nicely for that.
If it is in a business context where it is understood that confirmation is important, then it would be fine. However, in an informal e-mail I would find that sort of phrase brusque. If you are genuinely worried that the recipient won't get the e-mail then you could write something like: "I'm not sure I've got the right e-mail address for you, so could you let me know when you get this?"
Otherwise I would still suggest the "looking forward to" approach. Of course, much depends on your relationship with the person.