If you can't send but can receive email, then I do suspect this to be some sort of configuration problem. It is possible to remove and re-add an email account to Thunderbird without affecting other Thunderbird clients or losing data from calendar, contacts, etc if they are stored locally. You can remove the email account by going to Account Settings -> Account Actions -> Remove Account and then add it again by going to Account Settings -> Account Actions -> Add Mail Account and following the instructions. If you cannot afford to lose your calendar, contacts, etc I would export a copy just in case.

What is the antivirus on the non-working PC?

Problem not solved yet. I saved the the profile (as Zip File) from one of the working computers, removed the account on the computer playing up and re-setup the account and the saved profile, no joy . Removed the email account, removed the Thunderbird app. Down loaded the Thunderbird app and told the app to set up the email account from scratch. The profile from G-Mail, address book and current emails seem to have down loaded correctly. Emails are being received as normal. However a new sent email just sits there trying to send with out result.

In answer to the question what email protection is used "Malwarebytes" on all three computers. So that avenue may not be a problem. I use Kaspersky password manager on all computers, however there appears to no problem with accessing other sites.

I am able to take screenshots, so which screen shots may assist. Cheers, Amanda

To test if MWB or Kaspersky are interfering, run Windows in safe mode, which bypasses startup apps. An external password manager might affect authentication if it's entering mail passwords automatically.

Just to confirm, state your smtp settings (should be smtp.gmail.com, 465, SSL/TLS, OAuth2, email address).

The saga continues. I have three desk top computers all running the same programs in the main, for different purposes. all 3 run Windows 10, Thunderbird, Firefox, G Mail, same antivirus and password programs.

Two receive and send emails with no problem, the third will not send emails using Thunderbird. This happened overnight with no inputs from me. However I am able to send the email by opening Gmail using Firefox. I have opened the faulty computer in safe mode and tried to send using Thunderbird, same result .

I have re-examined the settings between the computers and have found the following smtp server settings, Description: Google Mail Server Name: smtp.gmail.com Port: 465 Default 465 Connection Security: SSL/TLS Authorization : OAuth2 User Name correct

On the computer not sending emails I note the following difference, Port: 465 Default 587 I have tried to change the default setting to see if it has any effect, but I have been unable to this.

As previously advised I have saved the Thunderbird profile from the working computer as a zip file and downloded it to the non working computer no change. I have deleted Thunderbird, reloaded it and let it do the set up for Gmail no change. It appears all settings transferred correctly except for the port setting.

Your advice please, Amanda

The saga continues. I have three desk top computers all running the same programs in the main, for different purposes. all 3 run Windows 10, Thunderbird, Firefox, G Mail, same antivirus and password programs. Two receive and send emails with no problem, the third will not send emails using Thunderbird. This happened overnight with no inputs from me. However I am able to send the email by opening Gmail using Firefox. I have opened the faulty computer in safe mode and tried to send using Thunderbird, same result . I have re-examined the settings between the computers and have found the following smtp server settings, Description: Google Mail Server Name: smtp.gmail.com Port: 465 Default 465 Connection Security: SSL/TLS Authorization : OAuth2 User Name correct On the computer not sending emails I note the following difference, Port: 465 Default 587 I have tried to change the default setting to see if it has any effect, but I have been unable to this. As previously advised I have saved the Thunderbird profile from the working computer as a zip file and downloded it to the non working computer no change. I have deleted Thunderbird, reloaded it and let it do the set up for Gmail no change. It appears all settings transferred correctly except for the port setting. Your advice please, Amanda

The gmail smtp works on either port 465, SSL/TLS, or 587, STARTTLS. Check that you don't have port 465 with STARTTLS or port 587 with SSL/TLS.

Check that there is no imap:// or smtp:// entry in Saved Passwords in Settings. There should only be a single oauth:// token.

Removing the account and the entry in Saved Passwords, restarting TB, then adding it as IMAP, will not lose any mail or contacts.

In case there's some error in the profile, you could test in a new profile, starting from Help/Troubleshooting Info, about:profiles. The original profile is not affected.

Problem Solved Thank you. The problem was the confusion in configuration of the smtp ports. Now changed to port 465 as SSL/TLS, and 587, STARTTLS. I also removed any imap:// or smtp:// entry in Saved Passwords. Many thanks, Amanda