Email spoofing refers to the sending email from one source, but making it appear message was sent from a different source. The email comes from spam@spam.com but it appears to be from you@yourdomain.com. Also, making the email appear to come from an unknown user within your domain name. For example, the message appears to be idontworkhere@yourdomain.com.

OK, it does not mean you should immediately think a hacker has taken control of your network or email. It simply means someone has created a workaround to send email as another domain. Not to minimize that this can be extremely annoying as it typically yields many return receipts. I advise taking action to combat this by adding an SPF record to DNS.