Cannot Send Mail

I was having the same problem but solved it doing the following: I already had an existing Yahoo email address, so I went to Settings, Mail, Accounts, and deleted my Cox email account on my iPhone.(changing the IMAP / SMTP settings on my phone didn't solve the problem).  I then went to the Add Account section and selected Yahoo from the choices (iCloud, Microsoft Exchange, Google, Yahoo!, AOL, Outlook.com, Other). I signed in with my cox.net email & password and was able to get my Cox mail through the Yahoo mail on my phone. Since I already had a Yahoo mail account on my phone, my Cox mail is listed in the accounts as "***********@cox.net. I set my wife's iPhone up for her Cox mail and it is listed under Yahoo mail, since she didn't have an existing Yahoo email account already. Of course, this is all contingent upon you already having made the Cox / Yahoo transition outlined in the instructions provided previously by Cox.