From the view of economics, the authors analyze the disadvantages of Spam mail. For this purpose, we introduce a production function model and estimate the effect of Spam mail. As a result, we find that the existence of Spam mail decreases not only the level of the GDP (Gross Domestic Product), which is about 464 billion yen (0.1% of the Japanese GDP), but also reduces labor productivity. This implies that the existence of Spam mail directly and indirectly damages the entire economy.??Therefore, we suggest a full-fledged introduction of "Outbound Port 25 Blocking'' to all ISPs (Internet Service Providers) and "Sendmail Authentication'' to all firms as effective Spam mail countermeasures.