Cox Emails not going through Google Mail

This just started happening this morning. I keep getting error messages. Following are two of many messages I received. I x'd out email addresses for security.

Mail Delivery Subsystem <mailer-daemon@googlemail.com>Delivery to the following recipient failed permanently:

  xxxxxxxxxxxx@gmail.com

Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the relay smtp.cox.net [68.1.17.8].

The error that the other server returned was:
535 5.7.0 ...authentication rejected

Why is this happening and how can it be fixed. I am very upset?