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Regardless of the specific error, 451 is in the range of transient errors. An RFC-compliant sender should be capable of retrying the operation, after a suitable delay. It is quite possible that the receiving server either has a temporary space problem (queues full, throttling in effect, in the middle of being DOS-attacked) or that's the code they send for Greylisting.
Your PHP app must be capable of dealing with a 451 error code.
Turned out the server ip was on the Spamhaus PBL and thus needed SMTP Authentication enabled. Thanks Charles and Jim for questions and info getting me looking in the right direction.