It is my understanding that 4xx errors are client errors, and 5xx errors are server errors.
Your understanding is quite incorrect. See RFC821 and RFC2821 for details. 2xx replies are 'success,' 3xx are 'continue,' 4xx are 'transient failure' which means the same transaction could be successful if tried again later, and 5xx are 'permanent failure' which signal that the message should be bounced and not retried.
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Bill Cole [EMAIL PROTECTED]
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