As anyone who has been on the internet for a while knows, e-mail has a big problem with Unsolicited Commercial E-mail (UCE), also known colloquially as spam. Most of this problem boils down to the fact that the SMTP protocol does not do any validation message senders. While properly configured mail servers will validate their users prior to allowing them to send e-mail from their account, the protocol itself does not prevent random machines on the internet from sending mail from arbitrary users and domains. This allows spammers to send forged e-mails through misconfigured mail systems or simply send the messages themselves directly to the recipient mail server from VMs at hosting providers as well as compromised desktops and servers.
Luckily, there are steps that can be taken in order to detect or limit the spam directed to your system. A few common approaches are:
