lmtp should give temporary failure for mailbox unknown

[Ramprasad]

This is actually related to my previous question ... 

I just configured my cyrus server to accept mails directly from my
remote postfix server  over lmtp 

Everything seems to work fine but sometimes lmtp gives strange errors
like 

 to=<X>, orig_to=<X>, relay=202.162.229.40[202.162.229.40]:24,
delay=0.26, delays=0.25/0/0/0.01, dsn=5.1.1, status=bounced (host
202.162.229.40[202.162.229.40] said: 550-Mailbox unknown.  Either there
is no mailbox associated with this 550-name or you do not have
authorization to see it. 550 5.1.1 User unknown (in reply to RCPT TO
command))


  When actually the mailbox is valid. The same mail sent again reaches
the mailbox without problems. The cyrus server is highly loaded most of
the time.


I want lmtp (over tcp) to give tempfail instead. I am already ensuring
mbox exists at smtp level


Thanks
Ram