Cox email censorship?

I've been trying to send an email from my Cox account to another Cox account and when I do, I get:

An error occurred while sending mail. The mail server responded:  <xxxxxxxxx@cox.net> message rejected. Refer to Error Codes section at www.cox.com/.../email-error-codes.html for more information. AUP#CXSNDR. Please check the message and try again.

The email is long and has the following link in it to a government website:

https://www.azleg.gov/FormatDocument.asp?inDoc=/ars/33/01804.htm&Title=33&DocType=ARS

When I remove the link, the email goes through just fine.

If I send a short sentence with this link, it goes through just fine.

But long text with this email and I get the error.

When I call support, they tell me that I need to spend $10/ month on a support plan and they will help "fix" my Thunderbird email client. Rep says that if the problem ends up being on Cox's end that they will not give me a refund.

Is this a ploy to make more money from us?

I am baffled why this would be a problem on my end.

Also today I tried to respond to a perfectly valid commercial email message today and same error message. I have not played around with removing content, but this is unacceptable.