hi i am getting error when i trying to send mail out to external email address like gmail, hotmail, msn, yahoo

The error says 550 invalid recipent : user@domain.com

so to make sure that the mail server is working i tried sending mail to local address
which is local@nameoftheserver.com it works
but when i try to send mail to external address it get error listed above.

so i added my email sales@domainname.com to gmail to test whether the mail server is working correctly from gmail i can send email from sales@domainname.com and recieve both but when i try that from my server it doesnot work

my server is offshore and i am using enom mail server to send email
???

any help