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Exchange 2010: Unable to send email to certain domain after migration

We migrate from Mirapoint to Exchange 2010 last week.

Everything went smooth until yesterday a few users complaint that they are not able to send email to certain domain.

The error differ for every domain name.

The first error (eg user@contoso.com):
451 4.4.0 Primary target IP address responded with: "421 4.2.1 Unable to connect." Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts.

The second error (eg user@binary.com):
451 4.4.0 DNS Query failed

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1 Solution
This is probably a DNS problem.  Check you send connector and see how its trying to send out email.

Also you can try checking the domains in question by doing this:
at a cmd prompt type:

set type=mx

You should get a response of an MX record, if not then you need to look over your DNS to make sure its working properly.
amywahabAuthor Commented:
Hello sconstable..

I've run nslookup and able to get the record properly.  
how are you routing your emails ?

through dns or through smart hosts ?
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amywahabAuthor Commented:
Hello Mkris9,

through DNS.
Couple of things you can do here.

1. Check message tracking logs to get a more clearer picture. (May be its your outbound anti-spam filtering)
2. Create another send connecter to route emails to this domain in question through your ISP's SMTP server.
Pls can you also post the message logs as per (1)
amywahabAuthor Commented:
Problem Solved.

Create another send connector to route email through ISP as per Mkris9 advise.


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