Sendmail host name lookup failure

[Cameron Beattie]

I have a number of undelivered messages in the mailqueue.  The mailqueue 
reports: Deferred: Name server: yahoo.com.: host name lookup failure. I 
attempted to send these emails from a remote client (Outlook Express) and 
cannot understand why the domain name can't be discovered.

nslookup yahoo.com.
Server:    210.55.12.1
Address:    210.55.12.1#53

Non-authoritative answer:
Name:    yahoo.com
Address:    66.94.234.13

echo test | sendmail -v user at yahoo.com
<snip>
250 2.0.0 <.ID.> Message accepted for delivery
</snip>

/etc/resolv.conf:
nameserver 210.55.12.1

/etc/hosts
127.0.0.1    servername    localhost.localdomain    localhost

Don't know if the following is relevant
service nscd status
nscd is stopped

Any help would be appreciated.

Regards

Cameron