smtp 451 error when setting recipient
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...

smtp 451 error when setting recipient
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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...
