CXSNDR error sending E-mail - is this permanently broken?

Since last night I have been receiving this error when trying to send E-mail:

An error occurred while sending mail. The mail server responded: 5.1.0 sender rejected. Refer to Error Codes section at www.cox.com/.../email-error-codes.html for more information. AUP#CXSNDR. Please verify that your email address is correct in your account settings and try again.

I use pobox.com as a spam filtering/mail forwarding service and this is set-up as my sender address in Thunderbird.

If I change my sender address to use the cox.net domain I can send e-mail but I don't want to use that address; I want to use the pobox.com address which has been working for many years. I called Cox tech support this morning and was unable to get a straight answer as to whether Cox recognizes this as something that's broken and needs to be fixed. Can anyone from Cox answer this please?