Just tried again and got it back again with the same error, only sending it to cox.help@cox.com which is the link you put in your emails to me. I have had cox email for years, you can see by my account with you and never had issues until this last update that cox did and it appears I am not the only one with issues.
Your message did not reach some or all of the intended recipients.
Subject: test
Sent: 3/6/2018 5:29 PM
The following recipient(s) cannot be reached:
'cox.help@cox.com' on 3/6/2018 5:29 PM
Server error: '550 5.1.1 <cox.help@cox.com> relaying not allowed'