Reject it during the SMTP dialogue

[J.J.Gallardo]

I'm surprise today with a test that i've do it on my smtp server (qmail
1.03):
I send an e-amil (1Mb) to an invalid user and qmail accept it a then,
send a reply to the sender (another megabyte) saying that the user is
unknown. Total = 2Mb of my lines used for no actions.
Is there a way to reject mail during the SMTP dialogue so don't accept
mail to "invalids and/or unknowns user's"?