Email spoofing is when a spammer sends out an email made to look like it is from a particular domain but in reality it is not. You may notice that you are being spoofed if you start receiving a large amount of bounce back messages that look like they were sent from an email address on your domain.
Fortunately though these spoofed messages will not impact your domain's standing with blacklists because the messages are not being sent from your email server.
Unfortunately these messages are being sent from a server that we have no control over and can not directly stop these bounces from happening. You can however set up a SPF record that can help stop some of these spoofings from occurring in the future. However not all mail providers check SPF records yet so there is still the potential of it happening again.
If you have any questions please feel free to contact support for information.