Friction

Stamps (remember those?) make direct mail work. Because it costs money to send a piece of junk mail, you'll think two or three times before you mail something to a million people.

Email, of course, is free.

Except it's not. The friction that slows down sending email to everyone all the time is the cost of all the people you'll lose. You might lose them because they unsubscribe, or more likely, you'll train them to ignore you. Worse still, you might just make them annoyed enough to badmouth...