Refresh MX lookups for smtp client?

On SunOS 5.8 with postfix 2.0.3, mails have been deferred due to a
missing A record for the recipient domain's MX host. The MX entry in the
DNS has been changed more than 12 hours ago, and nslookup on the postfix
machine shows the new MX record. Nevertheless, postfix keeps trying to
send the mails to the old invalid MX host, even after i have set up a
transport map entry plus stopped and started postfix.

How can i enforce that the new MX will be used in future delivery attempts?

Regards,

wolfgang