Most users receive at least a few messages every day, and many users receive dozens. Spam is the Internet equivalent of junk mail, but much worse. Internet users must frequently pay to download each message they receive, whether they solicited the message or not. Paper junk mailers pay almost all of the social cost of their activities, but spammers create significant costs for Internet Service Providers and message recipients. Junk mail on paper, furthermore, is strictly regulated by postal inspectors and strong mail fraud laws, whereas spam has been almost unregulated.
Reputable businesses realize that spam violates the cultural norms of the Internet.
Internet users can defend themselves against spam to some degree if they really work at it. The ultimate solution, though, will be some combination of technical improvements and appropriate legislation. Some people make a habit of printing out spam messages that they find particularly offensive, just to have them ready for such purposes. The key, however, is your letter. If you want to follow up with a phone call, wait about three days and then call in the morning, when deadlines are less urgent.