incorrect_user@correct_domain accepted

[Ketan Bajaj]

I have been seeing a problem:
when i sent an email to an incorrect user at a correct domain, the qmail
logs show that the message has been accepted.
But if i send the same email, incorrect_user@correct_domain through outlook
(microsoft exchange), the email bounces back immediately, with error message
shown below:
************
----- Transcript of session follows -----
550 [EMAIL PROTECTED] User unknown
************

Any idea why email does not bounce when sent through qmail.
thanks,
Ketan