in that case i don't think you need better rules in out look, but a better Business Model.
You need to set up some thing a Little more complicated. but also simple.
how about when you confirm some one has payed , you send them an Email with one of 100 random numbers in it, then they reply with the number in the subject title. and then you out look checks the Email title to see if it one of the 100 numbers and then sends the E-Book