I suspect that what you need to do is to choose "manual" configuration, and make sure that the username is only the portion before the sign "@": username, not username@cox.net
I think that should do the job.
Press on "manual", and the proceed.
Just in case, - as for the rest of the settings, here is what you should have (please, do yourself a favor and use IMAP, not POP3):
Incoming Mail Settings Server name: imap.cox.net
Port number: 993 with SSL enabled
Outgoing Mail Settings Server name: smtp.cox.net
Port number: 587 with STARTTLS enabled OR
465 with SSL enabled
For both servers, you can choose the option (SSL and STARTTLS) with "strict" [certificate checking]
and "automatic" for one more option (I forgot how it is called).