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email not reaching another domain

dear all, i have a puzzling problem.
first i will explain my network topoly.i have an exchange server 2003 in my lan.i have an ISA server.behind it there is a hardware firewall.between the ISA and the hardware firewall i have a mail filter server.my users are sendimg mails fine but in the pat week our purchasing department users complained that they are unable to send users to a specific company and they always get this error:
Action: failed
Final-Recipient: rfc822;pissaelkhoury@dhc.com.lb
Diagnostic-Code: smtp;
Status: 5.0.0

i tried to telnet on port 25 the specifeid MX  but no response.
when i talked to them they said that they are reciving normally from other comapnies.
and what is weird is that they can send to us but we cannot send to them.

when i checked the log on the mailfilter i saw a failure message stating that :failed to connect to host (ip) and then the name of the domain.

does anyone has a clue how to solve this. ?thanks
0
sparks2000
Asked:
sparks2000
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2 Solutions
 
Alan HardistyCo-OwnerCommented:
Please check your domain against my FAQ which might highlight a problem in your configuration / server setup:
http://www.it-eye.co.uk/faqs/readQuestion.php?qid=2 
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AkhaterCommented:
sparks2000

first I have to say it is the first time I see someone from my country here :)

second I did a checkup and I can confirm that the issue is not from your side the mail server of the company you are trying to reach is not reachable so nothing you can do about it.

to prove it send an email to them from gmail/hotmail it will bounce back
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AkhaterCommented:
"when i talked to them they said that they are reciving normally from other comapnies."

sorry i overlooked this part,

I ran a test on their domain at mxtoolbox.com and it gave results completely different than the ones in my nslookup

nslookup is giving 1 MX pointing to mail.dhc.com.lb
mxtoolbox.com is giving 3 mx the first pointing to mx1.cyberia.net.lb

so either they have changed their mail server and DNS didn't finish replicating yet or they have a serious problem of split DNS.

in both cases nothing you can do
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sparks2000Author Commented:
AKhater,glad to hear from someone from my country also:)
hope you can reach guru:)
thank you for your help and i agree with you that it is their problem .maybe they have blacklisted me .
thanks alahardisty also for your useful tools .they were really helpful.
for that i will split he points :)
thanks again for your support!
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