Mine

--- qmail-smtpd.c.orig  Tue Oct 10 14:04:28 2006
+++ qmail-smtpd.c       Tue Oct 10 14:02:16 2006
@@ -133,6 +133,7 @@
 void err_authreq() { out("530 authentication required\r\n"); }
 void err_denied() { out("530 Permission denied\r\n"); }
 void err_realrcpt() { out("550 sorry, no mailbox here by that name\r\n"); }
+void err_realmailf() { out("550 sorry, your mailbox must exist\r\n"); }
 void err_tlsreq() { out("550 sorry, STARTTLS required\r\n"); }
 void err_heloreq() { out("553 sorry, you must provide your HELO host (#5.7.1)\r\n"); }

@@ -746,6 +747,7 @@
     flagbarfbmf = bmcheck(BMCHECK_BMFNR);
   }
   flagbarfspf = 0;
+  if (chkuser && !realrcpt_check()) { err_realmailf(); return; }
   if (spfbehavior && !relayclient)
    {
     switch(r = spfcheck()) {