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can't send emails to some recipients

Posted on 2008-11-03
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SBS 2003, Exchange can't send emails to some recipients.

We couldn't send emails to a few recipients. I checked the queue and Exchange retries to send the emails for two days then send the 'Undeliverable' message from System Administrator back to the sending user.

user@domain.com on 02/11/2008 14:41
            The e-mail system was unable to deliver the message, but did not report a specific reason.  Check the address and try again.  If it still fails, contact your system administrator.
            <server.business.local #4.0.0 smtp;421 Service Temporarily Unavailable>

I started with checking DNS and it quickly turned out that on our server machine after checking mx records for the recipients domains we have problems with (nslookup; set type=mx; domain.com) I get this:

DNS request timed out.

I looked up the DNS name (not mx record) for this particular domain and it worked OK. Then strangely enough the email for the recipient from this domain went through. When I tried nslookup; set type=mx; domain.com once again it worked OK.

I had done the same with another domain we had the same problem with and got it sorted out, but I would like to get to the bottom of the problem.

Any ideas?

Regards,

Jay

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Question by:jbodys
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dhoffman_98 earned 2000 total points
ID: 22867370
It would appear that your mail server is looking for the MX record for the recipient's domain, and the DNS lookups are failing. This could be for various reasons.

First, if the recipient's domain is configured so that only one server might be offering the MX record, and that server is overloaded or otherwise can not respond to the query, then it would fail. However, your mail server is retrying it several times over a few days and you would think that at least one of those queries would go through.

Another issue could be if your network is not set up for redundant DNS queries... meaning that all your external DNS queries are being sent to a single machine that might not have updated information for that domain, or might have problems forwarding the request to the other domain's DNS servers.

If it is only happening with one domain, then I would think the problem is on the other end. If this happens with many domains and is only intermittent, you might want to check with the DNS resolution settings on your end and make sure that you have second (or even third) sources for DNS lookups.

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