Let me put it this way - there is case law that says that you have no expectation of privacy for garbage you put in front of your house for the garbage man to pick up. So the government can search through it all they want without a warrant. It's similar with email - unsecured email can be easily read anywhere along the path from your computer to the recipient. With encription though, there is a clear expectation of privacy, so if some law enforcement agency tries to use information obtained from encrypted email without a seach warrant, the defendant can have that evidence excluded from trial as an illegal search, violating your constitutional right. They would have to get a warrant from a judge to read your encrypted email, and that takes a lot of proof to show that you are most likely involved in a crime.