E-mail is an obvious threat to traditional mail. In fact, the Internet is making package delivery services more important. The importance of package delivery to the post office would have surprised Benjamin Franklin and other early postmasters, for whom the post office existed to send only letters, government and military dispatches and packets, which were bundles of letters, multi-page letters, and other printed materials typically weighing less than 3 pounds in total. In those days, the postal service was different in other ways as well. Probably not, but expect letters themselves, the way the post office works internally and the means of affixing postage to change.
People use e-mail today because it is easy and efficient.
But some of the same new technology that made e-mail possible is also making traditional mail easier and faster to produce. But the greatest disadvantage to e-mail is credibility. It is also trusted, because the recipient has more visual information about the sender, not the least of which is the sender's identifiable physical address and the sender's choice of physical material and content. Moreover, recipients often believe that if a sender has to incur significant costs to prepare a mail piece, that gives the sender more credibility and substantiality. Another change to watch for is the Postal Service's ability to track the location of letters and packages.