Can't send email to other Cox accounts

I am experiencing the same issue, which started some time earlier in the month of April 2019:  I am unable to send email from one Cox account to another Cox account when the mail originates from the Outlook app (or any of the accounts to themselves).  If I originate the mails from Cox Webmail, I can send mail to my other accounts (and to themselves), and I will see the incoming email in the Outlook app Inbox for the respective account. Thinking this is some kind of breakdown between the Outlook app and Cox mail, I tried setting up an IMAP account in the Outlook app, but same issue -- cannot self test or send mail to other Cox accounts if I originate in Outlook.  Changing to IMAP changes the incoming server, and the issue appears to be in the outbound (SMTP) path between the Outlook app and Cox.

All other email traffic to and from any address that is not cox.net seems to work.

I have three cox.net mail accounts associated with my cable account.  All PCs are Microsoft running Outlook 2013, using POP3 accounts set up per latest Cox directive.  I just set up an IMAP duplicate for one of these in my Outlook app to see if the issue was POP3 related, but as I said, no improvement.

Not a critical issue, but is an enormous pain when I want to forward stuff to my wife, etc.  Is this some kind of change in Cox security protocol??  Ideas please!!