Eliminating spam is one of the most important tasks of today's email administrators. There are two equally important facets to eliminating spam: preventing it from being sent by your server and preventing it from being delivered to your users. Of the two, preventing it from being delivered is often the hardest.
Strictly speaking, sender validation is not an anti-spam technique, though it is often regarded as such. One of the interesting details of the SMTP protocol is that the sender of a given message is not restricted to the address of the actual sender. A person sending a message can specify any return address, just as they can specify any destination address. In a trustworthy environment, where no one has a reason to hide his or her identity, this is not a problem. However, on today's networks, viruses, scammers, spammers, and so-called phishers all wish to hide their identity when sending email.
The reason sender validation is usually...