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I have a Exchange Server 2010. When sending email to particular domain, Exchange always get 4.3.1 error from the primary MX host.

My expectation is Exchange will fallback by sending it to secondary MX host. How long will the Exchange try primary MX host and then fallback to the secondary MX of the domain that I want to send to?Where I can change settings like this?

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It will never try the second server if it can connect to the server with the highest priority. It will only try the second server if it cannot connect to the first server.

The error appears to be a server resource error on the other side. Most times (not always) a different MX record will point to the same server via a different network path, so hitting the secondary IP address might result in the same error.

You should try and contact the administrator on the other end (Look at the SOA or WHOIS record) to see if they can help you with these errors.

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    Hi, about this behavior: It will never try the second server if it can connect to the server with the highest priority. It will only try the second server if it cannot connect to the first server. ---> can you point me to Exchange documentation that tell this kind of behavior? Thanks :) – masegaloeh May 7 '15 at 15:56

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