There are six reasons unsolicited commercial e-mail is such a problem:
Cost shifting. Sending bulk e-mail is amazingly cheap. With just a modem and a computer, spammers can send hundreds or thousands of messages per hour, and though that relatively minuscule cost of entry into the market is a potential advertiser's dream, it quickly becomes a nightmare for those who pay the costs of receiving it. The costs can range from the long-distance charges or per-minute access charges for dialing into an Internet service provider (ISP) to the cost of connectivity and disk storage space at the ISP and the inevitable administrative costs when the incoming flood outstrips capacity, resulting in system outages. These costs can be quite substantial: one major U.S.-based ISP estimates that spam costs it more than $1 million per month, accounting for nearly $3 per user per month. It is much harder, however, to calculate the cost of opportunities lost because of system outages, delayed services, and overflowing mailboxes.