Getting this error: CXSNDR There was a problem with the sender's domain. Your email failed authentication checks against your sending domain's SPF, DomainKeys, or DKIM policy.

The reason you are getting this is because several months ago Cox decided to implement an antispam service through Cloudmark that promptly broke everyone's ability to use Pop email when they weren't on the local network.  I have the exact same problem. 

What they failed to consider is that people who have been using the service for years are getting screwed over.  We are supposedly connecting directly to a cox mail server using SMTP authentication.   The PTR record for the connecting IP should have no relevance and until the pull their heads out and pay attention, we will continue to be screwed over. 

I do not use web mail, I use an email client to connect to the server.  Cox needs to have Cloudmark relax the restrictions when an authenticated user is trying to send mail.