Email address spoofing is an activity of illegally using other's legitimate email addresses to send messages. Generally, you can tell if an email was spoofed when you receive bounce-back emails that you did not send and can verify with the headers included that it did not come from your server.
The best way to prevent this is to create an SPF record and/or domain keys. These can be created automatically through cPanel under email authentication. You may receive a warning afterward, but it should go away within 30 minutes. If you believe your email was spoofed, but are not sure, you can forward a copy of the bounce back with headers to support@midphase.com and we should be able to verify.