The description for the CXSNDR error code is: "There was a problem with the sender's domain. Your email failed authentication checks against your sending domain's SPF, DomainKeys, or DKIM policy".
That sounds like a problem with your Cox account. Are you able to send any email with that account? Try using your Cox account to send a "This is a test" email, containing no links or attachments, to your Cox account. Then send that same email to your Gmail account. If you're unable to send email from your Cox account to either account, you can use your Gmail account to contact email@example.com. Be sure to provide your full name, service address, Cox email address and a link to this thread.
If you're able to send emails with your Cox account, you can perform tests in an attempt to determine the cause of the error. Try re-sending your original email, without the links, first to your Cox account, then to your Gmail account. Then, try sending a test email containing only a link to this Forum thread, first to your Cox account, then to your Gmail account, to see if you can send emails containing links.
Please post your results so that others may benefit from what you find out.
1. I send nearly all of my email every day using one or the other of my two main Cox accounts and they work fine usually.
2. I did as you asked, sent the link of this forum thread to Gmail and Cox accounts. That worked. My Cox accounts work 99.999% of the time. That's how I know these emails are being treated differently (when they are bounced).
3. An email sent this morning from one of my Cox accounts to the other, containing the links, did work.
4. Very interesting -- this morning I sent the very same email from Cox account to Gmail account, which failed as I reported before, but this time it went through. Nothing on my system changed, it is the identical email with the same links.
The only variable I can see is the Cox system algorithms for figuring out what it will block. I have had previous problems with Cox rejecting email that dealt with criticizing government overreaction to Covid -- no links in those emails -- so it appears the issue is within Cox, not something I'm doing.
Based upon the definition of the error message, which you provided above and which I had found previously also on the Cox site, Cox is indeed deciding not to send the email for reasons known to Cox.
I do appreciate your feedback and suggestions. Before posting here, I had run tests and I have the documentation showing Cox is blocking my email based upon its content (links or otherwise). Please let me know what my avenue is for filing a complaint.