Spam == bad data

In any software system, the only effective way to deal with bad data is to prevent it from entering the system.  User interface programs regularly do this by, for example, refusing to let you enter your name when you’re prompted for your date of birth.  If the data is guaranteed to be valid when it enters the system, then the rest of the program can operate without continually checking.  Can you imagine how much more complex and fragile the program would be if it allowed you to enter “George” for your date of birth?  What would happen if the program tried to compare “George” to the current date?

Email spam is bad data.  And once it enters the Internet mail system, it’s impossible to eradicate.  It gets passed from server to server, and then to the clients, wasting huge amounts of bandwidth in transmission and causing untold frustration to millions of users.  Why do we let it into the system?  The only effective way to cure the spam problem is to prevent spam from entering the system in the first place.  We have the technology to do this, but for some reason lack the will.  I simply cannot understand why ISPs and large companies have not come up with a system of trusted servers that guarantee no spam.  If somebody can shed some light on this, I’d be happy to listen.