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.