This may sound like quite a list but please work through it with me providing info because there could be several reasons for the problem or indeed more than reason causing the issue. It would also be useful to know a bit more about setup.

It is obvious that something is blocking access assuming nothing is wrong with the server settings. That could be the OS, Anti-Virus, Firewall, VPN, lack of cookies etc or combination of more than one.

Q: Windows 11 - do you have smart apps enabled or disabled ? Q: Did this occur after you updated to Windows 11?

Q: Do you have product like CCleaner or WiseCleaner on computer or Anti-Virus with 'cleanup' capabilities ? If yes, then do not allow those products any access to any Thunderbird profile file or folder as they may cleanup cookies or passwords or session data etc and that causes no end of problems. So please create exceptions to stop these products accessing Thunderbird profile.

Q: What Anti-Virus are you using? Q: Is Thunderbird listed as an allowed program in the Firewall ? The Firewall may be the default provided by Windows OS or the Anti-Virus package may be controlling the Firewall. If no, then exit Thunderbird and set Thunderbird as an allowed app in the Firewall. This is important. Windows OS tends to allow it's own Microsoft type products by default, so you wonder why eg: Windows Mail or Outlook work but Thunderbird doesn't.

Q: Are you using a VPN - maybe you are unsure ? Q: Does your Anti-Virus provide a VPN ? I use Norton 360 and discovered after an update, that Norton had installed and switched on it's own VPN and servers were not allowing connections. I also had good links opening in webpages that were ok but completely unrelated and inaccurate, so I resolved many issues by switching it off. Please check to see if it is possible you have a VPN working because google may not allow access to server because it thinks it is a potential hack.

Q: Is this a gmail account you tried to set up and it has never connected? Q: Is it a gmail account which did work and suddenly does not work? Q: Can I presume incoming and outgoing are both effected? Q: Is this problem with one gmail account or more than one? Q: Do you have other non gmail accounts working ok ?

Q: Does that gmail account have anything saved in Password Manager?

• Menu app icon > Preferences > Privacy & Security
• Under Passwords
• Click on 'Saved Passwords'
• click on 'Show Passwords'

Normally you would see incoming imap & outgoing smtp or oauth for the gmail mail account if authenetication is Oauth2. What do you see? Is anything showing for the problem gmail account username email addresses?

Q: Are cookies enabled?

• Menu app icon > Preferences > Privacy & Security

Under 'Web Content'

• Select 'Accept cookies from sites'

Was that selected or not ?

• Click on 'Show cookies' button

Do you see any mention of google ? eg: consent.google.com , google.co.uk , google.com

If nothing - and you have cookies enabled then something is deleting your cookies For 'Web Content'

• Click on 'Exceptions..' button
• Enter: https://accounts.google.com
• Set to 'Allow'
• click on 'Save Changes'

Another possibiity:

If you are developing eg: a website, maybe using Apache and XAMPP - if these programs are set to start up automatically then they might be interfering. Stop those programs auto running.

This may sound like quite a list but please work through it with me providing info because there could be several reasons for the problem or indeed more than reason causing the issue. It would also be useful to know a bit more about setup. It is obvious that something is blocking access assuming nothing is wrong with the server settings. That could be the OS, Anti-Virus, Firewall, VPN, lack of cookies etc or combination of more than one. Q: Windows 11 - do you have smart apps enabled or disabled ? Q: Did this occur after you updated to Windows 11? Q: Do you have product like CCleaner or WiseCleaner on computer or Anti-Virus with 'cleanup' capabilities ? If yes, then do not allow those products any access to any Thunderbird profile file or folder as they may cleanup cookies or passwords or session data etc and that causes no end of problems. So please create exceptions to stop these products accessing Thunderbird profile. Q: What Anti-Virus are you using? Q: Is Thunderbird listed as an allowed program in the Firewall ? The Firewall may be the default provided by Windows OS or the Anti-Virus package may be controlling the Firewall. If no, then exit Thunderbird and set Thunderbird as an allowed app in the Firewall. This is important. Windows OS tends to allow it's own Microsoft type products by default, so you wonder why eg: Windows Mail or Outlook work but Thunderbird doesn't. Q: Are you using a VPN - maybe you are unsure ? Q: Does your Anti-Virus provide a VPN ? I use Norton 360 and discovered after an update, that Norton had installed and switched on it's own VPN and servers were not allowing connections. I also had good links opening in webpages that were ok but completely unrelated and inaccurate, so I resolved many issues by switching it off. Please check to see if it is possible you have a VPN working because google may not allow access to server because it thinks it is a potential hack. Q: Is this a gmail account you tried to set up and it has never connected? Q: Is it a gmail account which did work and suddenly does not work? Q: Can I presume incoming and outgoing are both effected? Q: Is this problem with one gmail account or more than one? Q: Do you have other non gmail accounts working ok ? Q: Does that gmail account have anything saved in Password Manager?

• Menu app icon > Preferences > Privacy & Security
• Under Passwords
• Click on 'Saved Passwords'
• click on 'Show Passwords'

Normally you would see incoming imap & outgoing smtp or oauth for the gmail mail account if authenetication is Oauth2. What do you see? Is anything showing for the problem gmail account username email addresses? Q: Are cookies enabled?

• Menu app icon > Preferences > Privacy & Security

Under 'Web Content'

• Select 'Accept cookies from sites'

Was that selected or not ?

• Click on 'Show cookies' button

Do you see any mention of google ? eg: consent.google.com , google.co.uk , google.com If nothing - and you have cookies enabled then something is deleting your cookies For 'Web Content'

• Click on 'Exceptions..' button
• Enter: https://accounts.google.com
• Set to 'Allow'
• click on 'Save Changes'

1: I could not find a "Smart Apps" setting. This started happening in April 19 a couple months after I upgraded to Windows 11. No new apps were installed at the time. 2: No cleaner apps are installed 3: I am using Windows Defender for anti virus 4: Thunderbird is listed for both public and private in the firewall 5: No VPN 6: this is happening on several gmail accounts that all worked prior to April 19. I have not tried sending with them. Non gmail accounts are all working normally. 7: There is no "Menu App" button to look for passwords. The passwords are all saved in Thunderbird though. Authentication is set to Oauth2. 8: I cannot find a "web content" setting to look for cookies.

Update, I found the web content area in Thunderbird preferences. Google is in the cookie list and the passwords are there too.

This seems to be a Window 11 problem as using Thunderbird on a indows 10 computer with the same accounts works perfectly.

re:1: I could not find a "Smart Apps" setting. This started happening in April 19 a couple months after I upgraded to Windows 11.

"Smart Apps" a new thing in Windows 11. This is a good explanation. I suggest you give a read. https://support.microsoft.com/en-gb/topic/what-is-smart-app-control-285ea03d-fa88-4d56-882e-6698afdb7003

re: There is no "Menu App" button

the 'Menu app icon' is the one with 3 horizontal lines located on the 'Mail Toolbar' toolbar

Personally, I still like to use the 'Menu Bar' toolbar, in which case 'Tools' > 'Preferences' > 'Privacy & Security' gets you to same location.

re : Google is in the cookie list and the passwords are there too. that's good.

Please perform this test: restart computer in 'Safe Mode with Networking' - you will need to use an ethernet cable connection between computer and router. Then start Thunderbird.

Report back on results.

Restarted in safe mode with networking and Thunderbird accessed the gmail accounts normally. Don't know where to go from here

Tried to go to Smart Apps Control like following the instructions from the link above and it is not on my PC. I search for Smart Apps Control and get no results

Restarted in safe mode with networking and Thunderbird accessed the gmail accounts normally. Don't know where to go from here

This proves you have some software blocking the access and the most likely culprit is Anti-Virus disliking something and/or Firewall which may think a newer version is somehow a different program.

It also proves there is nothing wrong with Thunderbird nor gmail.

Exit Thunderbird First reset Thunderbird as the default email app. Perhaps select another app and then reselect Thunderbird

Then reset Thunderbird in Firewall. Access Firewall, block thunderbird, ok it Access Firewall, allow thunderbird, ok it Start Thunderbird. REport on results.

Followed all these steps and it is still bad.

It's a question of trying to determine what is setting up a block and that block appears to be for OAuth2 connectivity.

Defender Anti-Virus - set up Exclusion for the 'Thunderbird' folder. https://websiteforstudents.com/how-to-add-microsoft-defender-exclusions-on-windows-11/

Directory path If you have Thunderbird installed in default location: C:Users/Username/Appdata/Roaming/Thunderbird

Exit Thunderbird. Then do a temporary switch off of Real Time Protection. https://support.microsoft.com/en-us/windows/turn-off-defender-antivirus-protection-in-windows-security-99e6004f-c54c-8509-773c-a4d776b77960 Start Thunderbird REport on results.

Added the exclusion, turned off real time protection. Still no good.

In Thunderbird what are you using in proxy settings:

Menu app icon > Preferences > General Scroll to 'Network & Disk Space' section Connection....click on 'Settings' It will show current proxy settings. Please post image showing what is setup.

It is not letting me paste the screen print. The only thing checked is "Use system proxy settings" Everything else is blank.

Could you check the settings on computer for Lettings apps access your location. I think it's here: Go to Start > Settings > Privacy & security > Location. Is it switched on or off ?

I'm on Windows 10, but Mine is switched off.

Go to Start > Settings > Privacy & security > Email Is 'Allow apps to access your email' switched on or off?

I'm on Windows 10, but Mine is switched on.

It is possible gmail is not accepting because location is blocked or very different from previous. Same idea as if VPN is used, gmail thinks it's a hacker. Is webmail for gmail account still accessible ?

Please try the following which has worked for cases which only worked in Thunderbird safe mode - we have to rule it out.

• Menu app icon > Help > More Troubleshooting Information
• Under 'Application Basics' - Profile Folder - click on 'Open Folder'

This opens a new window showing the contents of your current in use profile name folder.

• Exit Thunderbird now - this is important.

Look for the following files and delete them.

• cert8.db - obselete file
• key3.db - obselete file
• pkcs11.txt
• secmod.db - obselete file
• session.json
• xulstore.json

NOTE: do NOT delete:

• key4.db
• cert9.db
• logins.json

Start Thunderbird New pkcs11.txt, session.json, xulstore.json files will get created.

test gmail

Tried all the above still bad. At some point I was advised to delete all the gmail passwords. I did this and now I get no secure connection errors. I do not see anyplace to reenter the passwords

Did you restart Thunderbird ?