Do you only have one BT mail account in Thunderbird? Have you updated/checked the stored password in Thunderbird for both incoming and outgoing? * Settings > Privacy & Security * Scroll to Passwords section * click on 'Saved Passwords' * click on 'Show Passwords' * Locate the outgoing smtp://bt account line for account where 'username' is the correct email address. * Is the passwords correct? * Right click on the smtp:// line and select 'Edit Password' * Completely clear all contents and then carefully type in the correct password. * Click on 'Close' Exit Thunderbird and wait a few moments for background processes to complete - then restart Thunderbird. Wait for Thunderbird to perform the initial check with server to download new mail. Then test sending.