Anyone can send email to anyone else, within seconds, at zero apparent cost.
That is the greatest strength of the Internet mail system. It is also its greatest weakness. Because the system is biased in favour of delivery, it is prone to abuse in the form of spam, viruses, and phishing scams. The very features that made email successful now threaten its viability.
To combat abuse we must add accountability. On a social level, legislative approaches such as can-spam attempt to punish spammers for their trespasses.
On a technical level, sender authentication protocols give computers new ways to automatically distinguish forgeries from authentic messages.