There are usually two servers involved. One for getting email (Pop or IMAP) and another for sending (smtp) . They can be different machines using different logins, so Thunderbird stores the login data for each separately. With Gmail in particular, the same login and password is used for both. Passwords can be stored in Thunderbird under Tools|Options|Security|Passwords->Saved Passwords, but with Gmail it can be complicated by their byzantine security. Have a look there, it may tell you what you need to know. Otherwise, go to your Google settings on their website and look for guidance on how to change the account password.