Email account unaccessable with other email programs

Somehow my wife's Cox email account has become unaccessable except through WebMail.  I have tried Thunderbird, MS Outlook and Mailbird.  My Cox email account works just fine, I set it up smoothly with Thunderbird.  I also created a new Cox email account for my wife, and Thunderbird had no problem working with that email account.  Cox tells me that it is a Thunderbird issue, but I have the same results using Outlook and Mailbird.  Since Cox seems stumped by this issue (I have spoken with them several times to no avail) does anyone here have suggestions?  Also, I tried to import mail and contacts into her Gmail account, Gmail couldn't access her account.