Spam also known as junk emails or unsolicited bulk email, is a subset of electronic spam involving nearly identical messages sent to numerous recipients by email. In short, it is defined as unwanted bulk messaging.
For more info, please visit: http://en.wikipedia.org/wiki/Email_spam
Most spam complaints are due to the following reasons:
- Open relays
- Unsecured wireless network
- Exploited CGI or PHP scripts
- Poor traffic quality
- Sending mails too often