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

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  • This error indicates that the origin email system may not have an SPF record configured to allow our SMTP server to send email using their domain name. If they offer secure SMTP connections you may want to try configuring your email client to use their outbound server to see if that helps.

    -Chris
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