Apart from possible interference by Norton Security, your smtp settings don't look right. 'Relaying not allowed' usually comes from trying to send mail on a server that doesn't allow insecure connections, which is what you have.

Try using smtp.cox.net on port 587, STARTTLS security, authentication = normal password, and User name = part of your address before the @.

https://www.cox.com/residential/support/imap-server-settings.html

Thankyou for your response. You could not see it but I have a POP3 server so I do not think this will work. All of the server settings for email worked until I update Thunderbird to 52.6.

For POP accounts, you can use the smtp settings I suggested, or smtp.cox.net on port 465, SSL/TLS security, authentication = normal password, and User name = part of your address before the @. The incoming POP server should be pop.cox.net on port 995, SSL/TLS security, authentication = normal password, and User name = part of your address before the @. According to Cox, insecure settings will not work after Jan. 25, 2018.

When sending stops working after a TB update, it usually means your firewall has to be updated to allow the new version.

Tried that and it did not work. Sounds like the cox change is suspicious. Guess I will call them and see what they say. Never had a problem with Norton b4. Will let you know what I find out.

be sure.

POP is incoming. SMTP is outgoing mail.

Thunderbird treats the two server as completely different.

You are very very luck to have never had a problem with Norton before. My experience with it that is without disabling significant parts of it you can not even add a new account to Thunderbird. Following every update it has to be told to allow the latest version access to the internet..

Look here https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#Norton_Security

thanks for your help. Cox was the culprit.. Got it fixed. was out of pocket for awhile but eventually got it fixed