sender rejected. AUP#CXSNDR

Occasionally I am unable to send email to valid email address.  Today when trying to send an email to a location in Canada, to an administrator of an Email Hosting Company (EHC) over there, and Cox cannot send.

Error:  <my email address> sender rejected. Refer to Error Codes section at www.cox.com/.../email-error-codes.html for more information. AUP#CXSNDR

Referring to Mail could not be sent by Cox.  Every now and then I run into this problem.  It is intermittent.

One other thing, I could not set my email address under settings so that if and when Cox responds I will get an email.  I use Outlook.  The system then logs into Cox with UserID and Password, and then sends my email via Cox's smtp mailer.  So I am actually not using Cox email at all, never have in the last 10+. years