Spam is any message or posting, regardless of its content, that is sent to multiple recipients who have not specifically requested the message. Spam can also be multiple postings of the same message to newsgroups or list servers that aren't related to the topic of the message. Other common terms for spam include UCE (unsolicited commercial email) and UBE (unsolicited bulk email).
The individuals who send spam are typically people who purchased or harvested a list of email addresses. They send messages from numerous different addresses to all areas of the Web. These addresses tend to be "forged", or "spoofed" to hide who actually sent them.
All mail originating from Yahoo! has an originating IP address embedded in the full address headers. This address helps to identify the sender of the email and track the individual to his or her service provider. If a message appears to be from Yahoo! but the originating IP line is absent, then it is not really from Yahoo!.