Unknown Recipient Domain

[Dan Lists]

When a user mistypes an email address domain (eg @monsant.com), the
message is sitting in the queue for days before they know about it.
We'd like to give them immediate feedback instead of making them wail.

I'm trying to have the outbound mail server permanently reject email
with an invalid sender domain.

I am using postfix 2.7-20100117 on FreeBSD 7.1 p10.

I have:

smtpd_recipient_restrictions =
        reject_non_fqdn_recipient,
        reject_unknown_recipient_domain,
        permit_mynetworks,
        reject_unauth_destination
unknown_address_reject_code = 550

This works if the domain exists but the hostname does not, for example
email to nob...@asdf.gmail.com:

Jan 29 11:49:27 outbound postfix/smtpd[65568]: NOQUEUE: reject: RCPT
from hostname[12.34.56.78]: 550 5.1.2 <nob...@asdf.gmail.com>:
Recipient address rejected: Domain not found; from=<d...@nowhere.com>
to=<nob...@asdf.gmail.com> proto=ESMTP helo=<hostname>

If the domain does not exist, it is giving a 450.  Here is the log for
an email to monsant.com:

Jan 29 11:48:23 outbound postfix/smtpd[65568]: NOQUEUE: reject: RCPT
from hostname[12.34.56.78]: 450 4.1.2 <nob...@monsant.com>: Recipient
address rejected: Domain not found; from=<d...@dan@nowhere.com>
to=<nob...@monsant.com> proto=ESMTP helo=<hostname>

How can I make postfix issue a 550 error when the domain does not exist?

Thanks,

Dan