Exchange 2007 Missing Mails

After we switched providers we have a problem that some Mails don't reach our customers and vice versa. We don't get any messages when our mails don't reach their target. Some customers get a message like this: couldn't find a mail exchanger or IP address. (#5.4.4)

DNS has been changed for two weeks now.. So IP should be the new one everywhere..

Need some tipps where to look..

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awhughesConnect With a Mentor Commented:

I'd always recommend doing a mail trace, using the Exchange Admin interface, just to see where things get to in the route.

Then go to the Exchange server and try sending a mail manually. Open a command prompt and type "telnet 25" (where is the IP address (MX Record) of a customer mail server that is rejecting mails). Try sending a short mail using the commands:
MAIL FROM: <your email address>
RCPT TO: <customer email address>
SUBJECT: <any text>
enter some text
end message by pressing a full stop on a line on it's own.

If there is a problem you should get some errors. These should be 3-digit SMTP error codes, which you can look up to find what the problem is.

Sending and receiving emails are very different things, suggesting that if they are both broken, then the change of provider should be the root cause. I would look at the DNS entry for your mail domain -I assume this is held by your new ISP?- and check that the Mail Exchanger record -MX record- is configured correctly with the IP address of your exchange server and that there is only one record, or at least that all IPs listed point to an exchange hub server. Replies may not be coming back because you no longer have SPF records in DNS.

Otherwise check if your new ISP is blocking SMTP. Some capture SMTP and redirect to their own servers, which will fail relaying for your messages.

Finally, maybe your exchange config is set to send mails direct to a smart host on your old provider?
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