I posted this over at cpanel forums with no response so hopefully someone here can help:

I'm attempting to email an account on my cpanel box (box2) from another box (box1, not running cpanel) using a php script but get the following error in my logs (box1) when doing so:

stat=Deferred: 451 rejected: temporarily unable to verify sender address (try again later)

The mail does not get delivered.. Is there a configuration I have to change or something so this will work?

I don't know if this is related but, we are having a problem with a couple of accounts on our CPanel server as well. Sending mail to an email account not associated wtih the domain name of the account, doesn't seem to work at all. I can't find that error in my logs though.. maybe I missed it.

Edit: The mail is being sent via PHP script.

this has something to do with the setup of dns, as we get the same errors and see them in our logs