What is your anti-virus software?

Try to start Windows 7 in safe mode with networking enabled.

Does the problem go away?

It still did not work in safe mode with networking I'm using Norton anti-virus. I tried turning off the email and networking features in Norton and that also did not help

I to help a friend who insists on installing Norton. I found I have to disable something called sonar to stop it making a complete mess of account configurations. I also have to stop scanning on encrypted connections and all the email functions. Still it needed regular maintenance in it's firewall to get to to allow newer versions of Thunderbird.

My guess is you have not actually completed to oauth process for the SMTP server. but I might be wrong. Do you have an Auth entry in your passwords in preferences for the outgoing server or just the incoming one?

I have OAuth2 in both incoming and outgoing in preferences.

I have turned off everything in Norton and it didn't help (Computer - Auto Protect and Sonar, Network - Smart Firewall, Intrusion Prevention, Email Protection, and Web - Browser Protection and Download Intelligence)

re :I'm using Norton anti-virus.

I use Norton 360, after an auto update, I had issues. I discovered that Norton had automatically switched on VPN. It was under 'My Norton' not 'Settings' I switched it off and all issues magically disappeared.

Please check this on your Norton.

Thank you all for the suggestions. I confirmed my Norton VPN is off so that is not the issue

Hello - bumping this up to see if anyone else has a solution or suggestion

All incoming and outgoing Authentication Method should now be using OAuth2.

We presume the smtp is using Oauth because it should be, but maybe it is still sending the old smtp password instead and this causes a failure.

Please access the saved passwords and remove the 'smtp' saved password for that email address/user name.

• Menu app icon > Preferences > Privacy & Security
• Passwords section
• Click on 'Saved Passwords'
• Do you have a smtp line for the problem email address / user name ?
• If yes - select it and click on 'Remove'
• Click on 'Close'

then Exit Thunderbird, allow a little time for background processes to complete and restart Thunderbird.

The only saved passwords are type Oauth so that is not the problem. I can send small emails. 17.9kb would not send, but removing some text allowed it to send

I found that logging into gmail from Firefox often hangs after putting the user/pass and clicking login. Then I open another Firefox tab and go to gmail and I see my inbox without having to authenticate, so the 1st tab has completed the login authentication, but does not go to the inbox screen

I have not had any problems logging into gmail from Chrome

Is there some type of timeout setting or email size setting?