Email "spoofing" happens when a spammer sends out spam to other people using completely unrelated servers and your email address as the return address. In this process, they are neither using your account nor using our server to send the email, but they forge the "reply to" address to be another address - in this case, yours.
It's effectively the same thing that happens if someone in California were to send a letter to someone in New York with your return address in Boston on it. If that letter wasn't deliverable, it would "bounce" back to you even though you and the post office in Boston had never seen it before. And there's nothing that you could do to stop it.
This end up affecting you in a couple of ways. Sometimes, spammers will address the emails to you and make them from you. This can get around spam filters that scan for bad "from" email addresses.
