I've also been having issues since Cox's move or "upgrade". All of my previous mail in my "SENT" box disappeared. I had several important chains in there that I can no longer retrieve. Also, sporadically, mail in my "INBOX" spontaneously disappears. Incredibly frustrating. I have been on the phone and chatted online with Cox to resolve this. I have several tickets open and it has been escalated a number of times. No satisfactory resolution. No explanation. They can not retrieve the lost emails (so I am out of luck with those) and the system has been buggy since October 2017 when I first noticed this.
Currently, my email's INBOX goes completely blank once in a while until Cox refreshes it and I have to wait while the current emails in the INBOX get reloaded. Just frustrating when I need to respond to one of them. I use this email account for my newly started business and it has become unreliable. I'll just switch to another provider since Cox can not tell me why this happened nor fix it. I just have to eat the cost of another set of business cards with a new email address on them.
Incredibly frustrating!