Email spamming, in simple words misusing the emails of a website is a common concern today. There are many ways by which this malpractice could be prevented. You might be familiar with 'captcha', one of the method used to prevent spamming. (For test cases for captcha refer: http://www.zyxware.com/articles/3308/websoftware-testing-checklists-cap…).
Drupal provides some modules like Spambot, Honeypot etc for blocking spam.
Spam Bot
Every website has POST and GET feature involved for continuous client and server communication. We would have got many unauthorized requests from the site to and fro which are categorized as spams. Spambots usually create fake accounts and send spam using them, although in many cases it would be obvious that a spambot is sending it.