Unable to send mail to cox recipients

I have used Cox as our primary email for many years, but lately my wife and I are having a real issue sending email.  Anytime we try to send email to an address at "cox.net", it fails.  (usually with no indication of the failure)  I have an iPad, iPhone and Windows PC and all fail in the same way.  Initially, I thought it had something to do with my iPhone, but the same failure happens in Windows.  We discovered it by accident, sending out a mailing to a group of friends.  All of the friends who have other service providers or gmail received the message, but anyone who had an email address "@cox.net" got nothing.  This is happening for all our email accounts.  At first I thought it might be a server problem (smtp.cox.net down), so I added smtp.east.cox.net, smtp.west.cox.net, and smtp.central.cox.net to my iPhone outgoing SMTP connections.  This had no effect.  At this point, we are both forced to drop Cox as our primary email and switch to Gmail.  

Has anyone else experienced this?  

Frustrated...