This is more a sign of the sluggishness of the legal system to follow the reality. More people use email now than regular mail, even for official things. Most companies even accept a pdf of a signed contract as an attachment these days.

The thing is though, if email was treated in a way similar to Federal mail, the whole debate about encryption and due process could be handled by laws that already protect privacy. For some strange reason, because it uses a computer, we lose many fundmental rights that we already had with Federal mail. In a certain way email is a service so fundamental to society now, that it becomes a necessary utility - so many things can't be done without an email address today.

Yeah, it's usually called an "electronic communication" in legal stuff, and cannot be substituted for the mail. I've seen this come up in organization bylaws that say that things need to be done via postal mail, and they need to change their bylaws to allow e-mail to be used instead.