Read the forum guidelines
If I try to log into my Lowes account and numerous other accounts I am continually flagged to add my email address back
I don't see how that would have anything to do with Cox email. Even if Cox email stopped being a thing, you should still be able to use the email address as a username. I would suggest contacting Lowes and ask if they know why the account is doing that. Maybe your not getting the account verification email? Have you checked your spamm?
I assume you can log into you Cox email account OK?