Such issues are often the result of security software and anti virus programs to test, restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 8, Windows 7, Windows Vista, Windows XP, OSX

Is this a new account or one that was running ok previously?

One reason for getting a password error message could be that the password is good but not for the server settings used.

So check server settings: Tools > Account Settings > Server Settings for the mail account Username = full email address

Please post info: In thunderbird

• Help > Troubleshooting Information
• make sure 'Include account names' is NOT selected.
• Click on 'copy text to clipboard'
• Then in the forum paste the info into a reply.

Edit /remove all info on fonts and printers but nothing else.