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I have setup postfix to send outgoing mail, It works fine for most of the cases except for some e.g where it gives me a 450 error, even though the email address is valid.

This is the exact error I get Sender address rejected: Domain not found (in reply to RCPT TO command))

Any suggestions on how I can rectify this?

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What's the sender address (or more precisely the domain part of the sender address) your MTA is offering to – joschi May 12 '10 at 7:06
it is – Shiv May 12 '10 at 16:32
Domain might be wrong according to this URL: (shouldn't it be – weeheavy Dec 9 '10 at 14:10

Just to check, neither a dig to nor to gives any answer. An whois to also returns no match.

You are sure that the subdomain is valid in your environment?

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