lmtp rejecting mails for valid mailboxes

[Ramprasad]

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. 

  What could be the reason. The cyrus server is highly loaded most of
the time could that be a reason. 

Anyway I dont want lmtp failures to bounce the mail , can I send a 450
instead of 550. Because I am already ensuring the mailbox exists before
accepting the mail.
How do I configure this on postfix or lmtp ? 


Thanks
Ram