SPAM generally contains a false sender address (return-path), this is "Sender Address Forgery". This is a problem if the email address(es) that are forged in torrents of SPAM belong to you. You will know this is the case when you receive the bounces and out-of-office replies from those SPAM emails.
SPF or Sender Policy Framework allows you to protect your domain from this abuse.
SPF is a record in the Domains DNS zone that allows an email recipient to verify that email originated with the server(s) designated by the domain owner.