Stamps ! Stamps ! Stamps !
When you send a piece of email, you need to attach postal stamps to demonstrate you have paid the right amount of postage to the dimensions and weight of the envelope or package. The first such stamps to be utilised on the planet were in the uk and Ireland. Prior to the introduction it was the receiver of the email which had to cover the postage once they received it. This made it much easier to send email when it was compensated for by the sender and has been a means of making certain all mail was sent.