Unable to Send with thunderbird

Hello, I use thunderbird to get and send emails. I can still retrieve my email with Thunderbird. When I try to send an email I keep getting an error message.

Sending of the message failed.
An error occurred while sending mail Outgoing server (STMP) error.
The server responded: cxr-obgw-5002a.stratus.cloudmark.com cmsmtp 15.181.96.243 blocked. Refer to Error Codes section at www.cox.com/.../email-error-codes.html for more information. AUP#CXDNS.

I however looked up this code:

CXDNS

There was an issue with the connecting IP address Domain Name System (DNS).
The Reverse DNS (rDNS) lookup for your IP address is failing.

Confirm the IP address that sends your email.
Check the rDNS of that IP address. If it passes, then wait 24 hours and try resending your email.

I have been trying to find how to contact cox by email but I am unable to find anything. Can anyone help me?