smtp 451 error when setting recipient

Question

I'm troubleshooting a php application which uses smtp to send an email occasionally. I don't know when the last time an end-user did this successfully (possibly it's been a couple weeks), but I am now getting a 451 error at the point where the reciepient is being set. What could be the trouble here and/or what should I check into?

This is on a dedicated linux server is hosted elsewhere. I'm not aware of anything changing on the server, but I wouldn't be wholly shocked if it did...

Answer 1

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.

Answer 2

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.