Quite simply, electronic mail. While physical mail was a crucial means of communication for centuries, email has largely replaced "snail mail" as the primary method of sending messages over long distances. Starting in the 1990s email phased out physical mail for both casual and professional communications. The fact that files can easily be attached to emails made the service indispensable for the sharing of information and for collaborating on projects. That and the fact that email can be mass-distributed instantaneously to multiple recipients (and is completely free) confirmed its superiority over snail mail. That being said, legal documents such as original copies of secure forms are still moved back and forth physically for legal reasons.
Email accounts are password-protected, information is digitally secured and email addresses can be fairly innocuous pseudonyms with little link to the owner of the address.