Intermittent problem sending cox email through gmail

Like many people, I use gmail exclusively to send and receive all of my cox emails.  It is configured correctly with the right passwords, TLS, ports, etc (I have confirmed with Cox).  Gmail dutifully grabs all of my Cox emails, etc. All of a sudden, however, while I can still RECEIVE all cox emails at gmail, SOME (not all) of my OUTGOING emails come back with the following error message.  I am out of settings to try.  If you have a fix for this, please let me know.   Cox blames it on gmail.

Message not delivered

You're sending this from a different address or alias using the 'Send mail as' feature. The settings for your 'Send mail as' account are misconfigured or out of date. Check those settings and try resending.

The response from the remote server was:

554 cmsmtp blocked. Refer to Error Codes section at for more information. AUP#CXBL