The hiccup I mentioned above is usually a short interruption in your connection through the internet from your iMac mail app and the Cox SMTP server. That’s the server that sends email out for you, which is separate from the POP/IMAP server that receives email for you and delivers it to your inbox. This can happen sometimes if there’s a ‘hiccup’ in your internet access while the mail app is trying to connect to the Cox SMTP server.
It sounds like the mail sending settings got messed up. The error messages says something like “relay not permitted”. All the sudden one day, it stopped working right.
I have an iMac (High Sierra) and use the Mac Mail app with my cox.net email account.
