Lost email access to secondary Cox email account

For months now I cannot send an send an email message from the primary email account to the secondary email account.  Nor can I send one from the secondary to the secondary.  The email address is established in my contacts but when I type it into the mail address I am not prompted nor is it recognized.  All on Cox.  

sometimes it tells me that the user is unknown.

I have done updates of all software, rebooted endlessly,  scoured the internet for a clue.  Nothing that worked.  Is it a Cox update flaw/?  Do I have to delete the secondary account and start over?  Not being a real computer guy this mysterious and frustrating.