Helo command rejected: Domain not found

Which Postfix restriction generates: "Helo command rejected: Domain not found"?

>From the log on albatross.python.org:

Aug 21 15:07:07 albatross postfix/smtpd[15378]: NOQUEUE: reject_warning: RCPT 
from qmta09.emeryville.ca.mail.comcast.net[76.96.30.96]: 554 5.0.0 
<QMTA09.emeryville.ca.mail.comcast.net>: Helo command rejected: Domain
not found; from=<[EMAIL PROTECTED]> to=<[EMAIL PROTECTED]> proto=ESMTP 
helo=<QMTA09.emeryville.ca.mail.comcast.net>
Aug 21 15:19:42 albatross postfix/smtpd[11319]: NOQUEUE: reject_warning: RCPT 
from cat.lrt.ru[91.192.244.67]: 554 5.0.0 <cat.lrt.ru>: Helo command rejected: 
Domain not found;
from=<[EMAIL PROTECTED]> to=<[EMAIL PROTECTED]>
proto=ESMTP helo=<cat.lrt.ru>

The only warn_if_reject in main.cf are:
# fgrep warn_if main.cf
    warn_if_reject check_helo_ns_access 
cidr:/etc/postfix/nsbl.cidr.msg.outboundindex,
    warn_if_reject check_helo_ns_access 
cdb:/etc/postfix/nsbl.hosts.msg.outboundindex
    warn_if_reject check_sender_ns_access 
cidr:/etc/postfix/nsbl.cidr.msg.outboundindex,
    warn_if_reject check_sender_ns_access 
cdb:/etc/postfix/nsbl.hosts.msg.outboundindex,

# fgrep -i domain /etc/postfix/nsbl.cidr.msg.outboundindex 
/etc/postfix/nsbl.hosts.msg.outboundindex
(no match, just to prove there's no LHS of "Domain not found"

But according to

# host QMTA09.emeryville.ca.mail.comcast.net
QMTA09.emeryville.ca.mail.comcast.net has address 76.96.30.96

# host cat.lrt.ru
cat.lrt.ru has address 91.192.244.67

the hostnames even resolve. 

But:

a) Why would check_helo_ns_access result in a "Helo command rejected: Domain 
not found"
   error?
   I usually use reject_unknown_helo_hostname for that, but:
   #  postconf |grep unknown_helo_hostname
   unknown_helo_hostname_tempfail_action = $reject_tempfail_action
   
b) for a domain that does exist (ok, admittedly, NOW, it could have
   been differently 8 days ago)
   
-- 
Ralf Hildebrandt
  Geschäftsbereich IT | Abteilung Netzwerk
  Charité - Universitätsmedizin Berlin
  Campus Benjamin Franklin
  Hindenburgdamm 30 | D-12203 Berlin
  Tel. +49 30 450 570 155 | Fax: +49 30 450 570 962
  [EMAIL PROTECTED] | http://www.charite.de