AUP#CXSNDR error when sending email

I am a Cox Gigablast user in Phoenix, AZ.  Since Wednesday, May 20, 2020, I am no longer able to send email from Mozilla Thunderbird using a third party email (xxxxxxxxxxx@media.mit.edu).  This has been working for many years and I didn't make any software changes recently.  I also have all the latest versions and patches of Windows and Thunderbird installed.
The error I am getting in a popup after I send the email is

An error occurred while sending mail. The mail server responded:  <xxxxxxxxxxx@media.mit.edu> sender 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.

I am using the following SMTP config:   smtp.cox.net   port 465   Normal Password    SSL/TLS