Employers are also worried that email will be used within the workplace to harass or offend other employees. For that reason, most of the monitoring software available to employers -- and more than one-third of employers reportedly use such software -- allow employers to locate email with offensive language.

But the biggest concern that many businesses have is that archived email will come back to haunt them in court. Unlike a conversation around the water cooler, the email statements of employees can live indefinitely on in backup tapes of corporate systems. Unless a company has a plan to purge old backups -- and most don't -- archived email can be a gold mine for lawyers representing anyone that sues the business. For example, when government lawyers sued Microsoft over antitrust issues, some of the most incendiary evidence came from archived emails that documented statements by Microsoft executives about its strategy against competitors such as Netscape.

Similarly, in lawsuits alleging sexual harassment or discrimination, judges have permitted into evidence inflammatory emails of a racist or sexual nature as well as email requests to a human resources director on how to avoid a wrongful termination claim when firing an older employee.

By placing employees on notice that email isn't private, employers can try to avoid the creation of such incriminating emails in the first place.

The ECPA also permits an ISP to look through all stored messages, including email awaiting you in your mailbox or recently sent and received mail. Some ISPs temporarily store all messages that pass through the system. The ECPA normally prevents the ISP from disclosing the messages to others, but even here there are exceptions. Law enforcement officials, when armed with proper warrants or administrative subpoenas, can gather basic information about users from ISPs, including their names, and also gain access to the content of stored messages. Also, once the email reaches its destination, the ECPA does not protect against snooping at the recipient's mailbox.

Some ISPs, worried about their own liability for the email content, require subscribers to conform to an End User Service Agreement that further reduces the user's expectation of privacy with ISP-favorable terms. For example, the service agreement for one popular ISP states: "Service Provider has no obligation to monitor the Service, but may do so and disclose the information regarding the use of the Service for any reason if Service Provider in its sole discretion believes that it is reasonable to do so, including to satisfy governmental or legal requests."

Commonly used public key technology uses two keys: one that is unique and private and one that is public and freely distributed to all users of a particular system. These keys only work when matched -- what one scrambles, only the other can undo. These techniques can also verify the integrity of the data (that it wasn't altered along the way) and authenticate it (check to make sure the stated creator is the person who sent the message).

But successfully using encryption requires some foresight, because the person receiving the message has to be able to decode it. Two popular encryption standards are Secure Multipurpose Internet Mail Extension ("S/MIME") and Open Pretty Good Privacy ("OpenPGP"). Neither of these software products can decode the other's algorithms.

In the end, email's speed and convenience outweighs its non-private nature for most every day discussions. But you should think of it like a postcard, not a letter -- a message open to every eye along the way.