I think you are testing mail to outside world by sending mail to the "inbound" interface of your Symantec Brightmail Gateway (SBG); that's what the error "554 Recipient address rejected: User unknown", if coming from your SBG, indicates.
To test mail to the outside world, you need to connect to the "outbound" interface of SBG. Also note that only IP addresses that have been configured as allowed host will be able to connect. So if you are connecting to SBG outbound interface from an IP address that is not in the allowed list you will get a "554...Client host rejected: Access denied" error message.
To allow an IP address to connect to the outbound interface of SBG, please see the section titled "Changing Scanner outbound mail acceptance settings" in Symantec Brightmail Gateway 8.0 Administration Guide available here:
ftp://ftp.entsupport.symantec.com/pub/support/documentation/sbg_administration_guide.pdf
JDavis has kindly provided steps to send test mail using telnet. You may also find the following KB article for mail testing useful:
http://service1.symantec.com/SUPPORT/ent-gate.nsf/docid/2005090207185063
Regards,
Adnan