Cox Email rejecting American Express Emails

How do I get Cox to stop rejecting emails from American Express?
I do not have any rules or filters set-up.

Here are the email addresses being rejected:
DoNotReplyUS@welcome.aexp.com
AmericanExpress@welcome.aexp.com

I had this problem with one of my Cox email accounts a few years ago, so I switched to another Cox email account now the second Cox email account is also rejecting these emails.

I've had the same problem with Capital One and Humana.

Who do I need to reach out to in order to fix this issue?