Spam Relaying is a dishonest practice whereby bulk email messages are forwarded, or relayed, through someone else's outbound mail server in an effort to mask the original source of the email. Although spammers usually act without the knowledge or consent of the administrator whose server is being used as a relay, it is impossible to tell whether the spammer was actually given permission to use the server as a relay.
For this reason, Internet providers commonly blacklist any server that is identified as an open spam relay, even though you may be more a victim of the spammer than the spam recipients themselves. Fortunately, your account has built-in protection against spam relay as part of its sendmail package.