AUP#CXSNDR continues to be a problem

Over the past 2 - 3 months I occasionally get emails rejected with AUP#CXSNDR. I have contacted Cox Tech Support and they have not been helpful. Rep on my call today said the email is being routed through an international server due to an attachment. Sequence is as follows: I send email with picture to one recipient; they respond; I respond back and it is rejected. No spam, not groups, etc. The email I responded to was only 12 KB per my Outlook inbox and there were no attachments (the picture in my original email was not attached in the response sent to me). I use a Cox.net email ID which I have had for about 30 years. I send and reply to 20 - 40 emails each day. This is sporadic and I can not detect a pattern. It is also new - I did not have this problem until the last 2 - 3 months. 

I have looked up the AUP code which says my email failed authentication checks against my sending domain's SPF, DomainKeys, or DKIM policy. The intended recipients of my rejected emails are people I frequently correspond with (and normally have  no problems). Does anyone have any ideas how I (or Cox) can fix this? I'd love to send a rejected email to Cox and let them look into it but don't think it would go through (and I don't know who to send it to).