Your email provider administers your password. You need to work with them for a forgotten password. Most providers have a link on their email help page to reset passwords. To see the password you told Thunderbird to remember go to.. Options-Security-Passwords-Saved Passwords Click Show Passwords. Incorrect server settings also cause the password error. If you still have issues after getting a password that you can remember to need to get the current settings from your provider and make any needs changes in what you have in Thunderbird.