I am not going to be helpful either. While we can work through what the issue is post update, guessing what variable is causing your issues is just that guessing. One constant over the years however has been anti virus products blocking the updated Thunderbird. Another has been outdated connection settings that stop working after an update because the server is actually not secure, despite Thunderbird having used what are now broken encryption methods in the past.

But without concrete error messages and server names everything is just best guess.

Hi Matt:

Thank you for the quick reply. Unfortunately I will probably have to stop using Thunderbird and move everything to Gmail if I can't resolve this. I have been using Thunderbird for 20 years.

I use Norton 360 Virus Protection. I don't know why that would block outgoing Thunderbird email when I can receive and send emails using Gmail. It seems that Norton would block both Thunderbird and Gmail outgoing email if Norton has a VPN feature turned on by default that is causing the issue. If Norton does have that turned on, there is probably a way to turn it off and give that a try. I will have to contact Norton to find out how to do that.

If Norton isn't at fault, I really don't know what else to try. I can't find any way to re-set my SMTP password. If Thunderbird will not send email I will have to use either Gmail or Mailbird instead.

Thank you for your help. Tom

Lets be clear here. I am a fellow thunderbird user. Threats or promises of leaving the product or donating make no difference to what happens at all. I don't get paid either way.

You choose to use Norton. Perhaps the number one cause of mail clients (not just thunderbird) failing to get mail. Can it yes, does it. For some folk it blocks for days or weeks after every update until it gets a message from home saying the new version is all good now.

Is there a relationship between gmail in your browser and Thunderbird using an email account. The answer to that is NO! In the browser it is "just another" web site. In Thunderbird to connection protocols, transmission ports and the applications involved are all different. I have only used windows defender since I bought my new PC. It is not been an issue so far after months.

I notice you specifically mention a VPN. Another common cause of trouble. The VPN hides where in the world you are by pretending you are somewhere else. Email providers are very reluctant to allow you to send mail using their SMTP server if you are apparently in Moscow and your home according to their records is 1600 Pennsylvania Avenue NW in Washington, D.C. It raises alarm bells with them that you might have been hacked.

Follow the instruction here to disable Norton email scanning https://support.norton.com/sp/en/au/home/current/solutions/v57846078 Not that I do not believe for one minute their claim the problem is not their product.

This person has issues because of Nortons firewall apparently https://docs.microsoft.com/en-us/answers/questions/575527/accesshotmailthunderbird.html

Basically the oldest method is probably the best in your case. Update and then run the operating system in safe mode with networking to try and identify the problem software. Not your choice of anti virus is probably the most likely.

This page covers the whole gambit of disasters caused by anti virus products over the years. https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues