[Exim] Resolving Problem

Hello Exim List,

I'm receiving this resolve error:

"
May 5 08:40:16 ms1 exim[31290]: 2003-05-05 08:40:16 19CYhr-000275-00 ==
info@??? R=dnslookup T=remote_smtp defer (0): SMTP error from
remote mailer after MAIL FROM:<sysadmin@???> SIZE=2197: host
mail.dutco.com [64.55.218.57]: 451 4.1.8 Domain of sender address
sysadmin@??? does not resolve
"

Any idea what should I do? Our domain (qafac.com.qa) cannot received nor
send to this domain (dutco.com). Not only dutco.com around 10 domain I
see having similar problem.

We're maintaining our own DNS and I have both MX and PTR record to my
mail server (ms1.qafac.com.qa , 212.77.204.5).

I'm using exim-4.12-1

Any hint please? Greatly appreciated.

Thanks.

Sandro T. Rafael