E-mail is becoming more and more commonplace. It will eventually replace postal service all together, though it might take a while. Postal Service companies are already stating that they are not making as much money as they used to. Mail doesn't even run on Saturdays like it used to anymore.
Although e-mail is fast replacing the U.S. Postal service, there are things that people can't send through the internet. All physical things will still likely be sent through the postal service. It's a lot easier to get scammed through e-mail, and a persons email account has the potential to get hacked and forced to send viruses through email to everyone they know. That can't happen with real mail.
Email is only good for direct communications, but there are somethings that just have to go through the postal system. Until somebody develops a teleporter, there will always be a need for packages to be delivered and documents to mailed. Sure, email has cut down on some of the volume carried, but it want kill the post office any time soon.
For friendly correspondence and everyday messages, e-mail is probably better and way more handy. But there are things that the postal service does so much better than the internet. We will still want to be able to send birthday cards and Christmas letters. An e-card is just not comparable to opening real mail!