Email spoofing is when someone else (usually a spammer) sends out an email and fakes your email address as the "From" address. This activity can happen without use of your account or your email server.

What then happens is that if the email is rejected, it will come back to you (since you're listed as the 'from' address). You'll get the bounce back message, but it will seem as if someone sent it from your mailbox.

Like someone sending a piece of postal mail from Florida to an address in California with your return address on it in Massachusetts, you'd get a piece of mail delivered to you that you've never seen before but which "bounced back" to you.

Unfortunately, just like in the scenario above, there's nothing that you can do to stop it. Since your account and server aren't used to actually send the email, the only thing you can do to avoid being spoofed is to completely change your email address, which most people can't do for obvious reasons.

This article covers how this can happen to just about anyone:

http://msnbc.msn.com/id/13875762/

"Meanwhile spam keeps coming from fictitious people using my Internet domain, and there's not much I can do. Mark Seiden, an Internet security consultant familiar with such shenanigans (he was even able to guess correctly which companies the phony e-mails were hyping), says that it's a technically trivial trick to anonymously put someone else's domain on the mail header. 'It's like someone writing your name on the return address of an envelope,' he says. 'You can't stop them from doing it.'"