"premature EOM: unexpected close" & DNSBL in SendMail

Mails from some of the accounts (but not all) from a site (a business partner in S'pore of my office) were not received since yesterday.
Then I checked the sendmail log. Following lines showing the error was found:

Oct 24 10:14:57 server sendmail [11672]: l9O2EM2Q011672: collect: premature EOM: unexpected close
Oct 24 10:14:57 server sendmail [11672]: l9O2EM2Q011672: collect: unexpected close on connection from dmzxxx.yyy.com, sender=<zzz@yyy.com>
Oct 24 10:14:57 server sendmail [11672]: l9O2EM2Q011672: from=<zzz@yyy.com>, size=0, class=0, nrcpts=3, proto=ESMTP, daemon=MTA, relay=dmzxxx.yyy.com [111.222.333.444]

Lookup the problem from google and finally, thanks God, found lights to the solution. (http://www.outofcontrol.ca/2007/02/28/sendmail-collect-premature-eom-unexpected-close-solution/)
It is found to be something wrong with the DNS Black List being used in the sendmail configuration file.
--> One of the open DNSBL, relays.ordb.org, was no longer responding.

According to DNS Blacklist ORDB.org is shutting down the relays.ordb.org DNSBL has been shut down since 18 December 2006.

By removinig that DNSBL, mails from those accounts are received correctly.

Now we use 3 DNSBL in the sendmail:

• zen.spamhaus.org
• list.dsbl.org
• dnsbl.sorbs.net

I think checking the DNSBL sites from time to time to ensure they are working properly is necessary.

Don't really understand the connection between the error and the DNSBL terminations. The questions are:

• Not all mails from the same site cannot arrive. One of the account from that site can arrive successfully.
• The error only found these 2 days but the DNSBL of ordb.org has been stop working since 18 Dec 2006!! ??

Anyway, the problem solved.