After you changed the password, did you delete the old one in Options/Security/Passwords/Saved Passwords, restart TB, and enter the new one when prompted?

Hi - thank you for trying to help me with this.

Yes - I deleted all Passwords when I changed and used the Password Manager to saved the new ones.

Also - one of the three emails can receive emails but not send and the settings are the same across all three. Which is really odd.

I't not anything to do with the gmail settings as I can receive and send on my other (Apple) devices.

Is the non-sending account set to use an smtp server with the same authentication method and User Name as the incoming server? Select the account in the left pane of Account Settings, then Edit SMTP server in the right pane to compare settings. If this is a gmail POP account, check that access by less-secure apps is enabled in the account's security settings.

Thank you all for trying to help me with this problem.

I finally identified the problem: Google (Gmail) has decided that Thunderbird is a "less secure app" and automatically blocks the send-password command. Th error message only advises that the username and password were not accepted - rather than advise that because Thunderbird is deemed to be less secure - access was blocked.

The solution is to go into the Manager Your Google Account menu in Gmail and select the option to allow "Less secure app access". This had to be done for all 3 email accounts for them all to work.

All sorted now. Thanks again everyone - much appreciated !!

A better approach would be to generate and use an app password instead of enabling less secure access. Anyway, this is just a side note and something you may want to consider in the near future.

Thanks. I’ll try and figure out how to do that. Cheers. Simon