Sendmail - did not issue MAIL/EXPN - problem with one company.

A reciving mail problem.

We have one company that is trying to send us mail but cannot.  In send
mail log I see them connecting but the log file has the  "did not issue
MAIL/EXPN..." entry for every connection they try to make.

I know what the normal definition of thius error is. I need some help
(ideas) with the diagnoses.
I cranked up sendmail's logging level.
Now I get a "lost input channel" message just before  just before the
"did not issue" entry.

Found out yesterday that they cannot telnet to port 25 either.
I have been unable to duplicate that problem.
Everyone else's mail is coming thru fine.

We do not reject any mail that is sent to the gateways. You can connect
and send mail as flintstone.fred this mail will be accepted. Spam
filtering and invalid address filtering happens behind the firewall on
a different server.
I have added this company's domain to sendmail's access table. - this
did not help
I also added an entry for this company to hosts - no joy.

The company has an unusual DNS set up.
The two listed name servers for then do not return any data when I try
a reverse lookup.
Their PTR records are handled by nameservers that are different from
the ones listed in the forward zone.
We don't seem to be having problems resolving their mail server ip's
into names though. Both name and IP address appear in the logs.
..
This company recently purchased an appliance "Iron mail" for their
e-mail system.
They have not been able to send to us since they installed it.
I'd like nothing more that to throw the problem back over the fence and
tell them to fix their stuff but I need some good evedence that it's
not misconfiguration on our side.

Any suggestions of what to look at on our side?

.

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