I had the same problem with Cox email being setup on a mobile phone. I have been using a specific app that I like which I can enter all the emails that I need to check at one time
but when I tried to add my cox email account, it just wouldn't accept it.
I talked to cox cable tech guys and was on there for over 30 mins but they couldn't figure out why it was being accepted so we tried the default email program that is installed on the
phone which can't be uninstalled and it seemed to work but again I didn't want to use that because I like using a transparent theme but I decided to not add it to my original app.
So, the techs told me that some apps are created differently so it will accept some emails and not others. Hope that helps.