semi open relay issue

ok here is the issue

RCPT TO:
554 5.7.1 : Relay access denied [187 ms]

so far so good
but if i change example.com to a domain my mailserver handle it accept and SENDS! the mail

i got a standard install of ehcp and postfix
can someone pls tell my why in the blue moon garden it accept all email using domains that is handle with out asking for passwd