The 4th Amendment says that the government can't look at your stuff without a valid warrant. It also says that in order to obtain said valid warrant, the government needs to have a really good reason to suspect you of a crime. The Founding Fathers didn't have emails, but they were smart and able to envision a future where people would want their activities to be private. Sadly, the NSA has violated the 4th Amendment many times.
Law enforcement should be required to get a warrant for access to a persons e-mail just as they need to in order to get into a persons home. E-mails can contain very person and private matters just as a house does and the invasion of that privacy should not be allowed to invaded without proper authentication.
Emails as with any other correspondence is private property, if law enforcement feels there is probable cause to search through some ones email then they should get a search warrant. Electronic communication is more commonplace than ever and should have the same expectations of privacy and property put on it as all things granted by the constitution.