Relaying Problems using PO3

I recently installed a CISCO PIX 515E firewalll and immediately after, my users who access their email using POP3 getting the following errors whenever they try to send email to external emails addresses, “550 5.7.1 Unable to relay for abc@xyz.com”. I know this was caused by the installation of the router and it doesnt’t affect emails sent within the company. Any help with this will be highly appreciated.