Passwords can only be created or modified etc via the webmail account for that email address which you access/logon via a browser. Once the new password has been set up, you then update the stored password in Thunderbird. * Settings > Privacy & Security * Scroll to Passwords section * click on 'Saved Passwords * click on 'Show Passwords' Accounts usually have two entries one for incoming and one for outgoing. Username is useful as it usually is the email address. Pop starts with 'mailbox://' Imap starts with 'imap://' Smtp start with 'smtp://' Some accounts use Oauth, so will see a line 'oauth://' * Right click on relevant line and select 'Edit Password' * Completely clear all contents * Carefully type in the correct password You need to do this for all the relevant line for the account/username * click on 'Close' Exit Thunderbird, allow a few seconds for background processes to complete and then restart Thunderbird.