451 4.1.8 Domain of sender address xxx@domain.com does not resolve

Hi All,

I have several email addresses at one domain @someisp.net.au (including two user addresses and the support address) that I can no longer send emails to. I cannot send them from my domain @mydomain.com.au which is Exchange 2007 and not from my clients domain @clientdomain.com.au which is Exchange 2010.

Both have the mail stuck in the queue with the error "451 4.1.8 Domain of sender address xxx@mydomain.com does not resolve"

I'm not having troubles sending to others, neither is my client. The User at the @someisp.net.au domain isn't having troubles receiving from others either.

I can send from a gmail account to the @someisp.net.au domain

The guy I spoke to at the ISP was an absolute rude wanker who was really not interested in helping, so I can only troubleshoot from my end.

I've done some reading and thought it might be an issue with reverse dns lookup, so I now have a PTR record in a reverse DNS zone for the IP of my primary MX record that resolves to the exact host name of my primary MX record.

Both @clientdomain.com.au and @mydomain.com.au have their forward DNS zones with DynDNS and have primary and secondary MX records

What else can I do to resolve?
crystaltecAsked:
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Simon Butler (Sembee)ConsultantCommented:
The error is coming from the ISP in question. They are doing some kind of test to confirm that the domain is valid, which your server is failing for some reason. Unfortunately they are sending back a temporary error (4.x.x) rather than a permanent error (5.x.x) which tends to suggest their setup is flawed. Rudeness at ISP support isn't uncommon, they don't like to be told their setup is wrong.

As a temporary measure, use a smart host on a new Send Connector to send email via your ISPs SMTP server.

Then check that your PTR matches the MX record, resolves to the correct place and is also the FQDN on the Send Connector.

This has nothing to do with your own DNS server.

Simon.
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EMJSRCommented:
This sounds to me more like a DNS problem. Maybe add an additional DNS server to your mail server/dns server. So, if DNS and Exchange are hosted on the same server, then maybe add an additional DNS resolver/forwarder to your DNS config. Maybe Google's public DNS (8.8.8.8 and 8.8.4.4).

If you have confirmed that both ends are working separately and you can send from another source (such as Gmail) - which you say you did - then it is verly likely a DNS issue.

So in the DNS properties (dnsmgmt.msc) add additional DNS servers; usually your ISP's should be there. Please see the attached screenshot (found via Google search, might look slightly different your end, depending on your SBS version).

Then restart your DNS server service and you should be good.
example.PNG
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crystaltecAuthor Commented:
I was thinking it was a DNS issue.

You are correct, my Exchange server is also my DNS server.

But if the message says the domain of the SENDER address doesn't resolve, that would be referring to my address. My server should be about resolve my address so i'm guessing it's the receivers DNS that is having trouble resolving my address...

Also, shouldn't the root hints be taking care of DNS resolution for external addresses...
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crystaltecAuthor Commented:
Thanks, this is exactly what I was thinking and it good to hear someone come to the same conclusion.

Unfortunately our clients ISP is equally hard to deal with (stuck between a rock and a hard place!) so i've explained all this to our client.

When configuring a send connector, can you specify that the send connector only acts for a certain set of recipient domains rather than sender domains? That way we could send mail for @someisp.net.au via the new send connector that uses our ISP's SMTP and all others via the original send connector that uses our own SMTP...
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Simon Butler (Sembee)ConsultantCommented:
Send Connectors are only for Recipient domains, nothing to do with the sender.
Create the connector in the usual way, then you can add the domains to the Address Space list. Exchange uses the closest match, so a connector with example.com will send email for example.com, everything else goes out the *.

Simon.
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