I also have this problem using the Apple Mail client on 2 Macs in the house using 2 Cox email addresses, and using Kmail email client in Kubuntu Linux. These mail clients can send via other smpt servers, but not through smtp.cox.net. Also, an iPhone fails to send when using house WiFi, which routes through Cox internet services, but the same iPhone succeeds sending through AT&T mobile phone service when the WiFi is turned off on the phone. This started yesterday.
Outgoing mail server: smtp.cox.net port 465, with SSL selected, user name and password unchanged from when it worked. And, the username and password work on the iPhone when sending through AT&T mobile phone service.
Is smtp access being blocked when using the Cox internet service?