Issues with Email
The biggest problem with email is there is some debate as to whether one actually has an expectation of privacy. Most people just look at email as an electronic version of mail, which is understandable, of course. The issue is that it isn't really treated in the same way as a letter.
The contents of an average letter are not read at any point in the chain. You can assume that only the receiver will see a private letter without really stretching logic. But what about emails? The average email is sent across a number of servers, scanned and effectively “read" in an electronic sense for spam and ad filtering and then delivered. Emails, therefore, are seen as a somewhat special case. A bit of a Schroedinger's cat, stuck somewhere between correspondence and data.
Emails are also stored in multiple locations. You can have a letter shredded or allow the contents of a phone call to disappear into the aether (depending on phone recording laws in your state). On the other hand, there will be copies of your emails available in your sent mail folders, the recipients' inboxes and the servers of the mail providers. The fact that they have indefinite access to this information adds in another layer. Add in the fact that most email providers and Internet service providers have terms of service that allow them to monitor your usage, and it becomes harder to argue a clear presumption of privacy with your email, at least in the same way that other communication is protected.
Because of these issues preventing a clear presumption of email privacy, special laws usually regulate it.