Check your saved passwords in Thunderbird: Thunderbird menu: Tools: Options: Security: Passwords tab: Click the "Saved Password" button. In this "Saved Logins" window you will see the incoming and outgoing servers for each of your e-mail accounts. I recommend clicking the "Remove All", click "Close", then restart Thunderbird. When Thunderbird restarts, it will ask for the password for each account when it tries to check the e-mail. See my snip below. Type in the password, and make sure that you check-mark the box that says, "Use Password Manager to remember this password." Thunderbird will also ask this when you send an e-mail the first time for each e-mail address. If you still have problems with Gmail, then you should check the e-mail server settings in Thunderbird for Gmail: Thunderbird menu: Tools: Account Settings: Select your Gmail account on the left side, then "Server Settings" under it. On the right side are the settings. Compare them to what Google says they should be: [https://support.google.com/mail/answer/7126229?hl=en Read (and send), Gmail messages on other email clients using IMAP] If there is still a problem, then perhaps it is because you need to set a [https://security.google.com/settings/security/apppasswords Google App password] for Thunderbird. See [https://support.google.com/accounts/answer/185833 Sign in using App Passwords] for more info about this.