Do not delete any accounts. Generally in Thunderbird, it tries the stored password and prompts you to enter a new password when it falls to connect using the stored password. Alternatively, you can go to Tools - Options - Security - Passwords -> Saved Passwords and delete the old passwords. Thunderbird then needs to ask you for the password and will do so when it needs it. With Gmail, you need to set each account up with both incoming (usually IMAP) and outgoing (SMTP) . If you have an account where you're using two different passwords, it seems most likely that you're using one SMTP "account" for both of the incoming accounts, that is, you're using account A's SMTP for both account A and account B. I'd check for this and then add an SMTP account for the one that doesn't have its own. You may actually find that you already have two or more SMTP accounts, but haven't correctly associated each incoming account with its corresponding SMTP settings.