Are you talking about a 'Primary Password' because that would be the only Password asked by Thunderbird OR are you talking about Thunderbird tries to synch with server and server is rejecting password - a mail account password ? Test: Are you able to logon to webmail account using password ? Please confirm the known password at least works for access to webmail. If you have recently enabled a VPN then most servers will reject attempts to access accounts as they think you are a hacker. Either switch off VPN or create a split tunnel for Thunderbird. Please note there have reports of VPN being auto enabled by some Anti-Virus products, so check this out. Is mail account password stored in Thunderbird ? Please confirm password stored correctly in Settings > Privacy & Security > Saved Passwords > Show Passwords. Much depends upon what server you are connecting to. Please tell us is this a gmail, yahoo, outlook365, comcast, xfinity, etc server ? Microsoft (outlook365 server) have altered things recently when it comes to passwords - password for Microsoft account must now be the same for outlook.com, live.com etc accounts - all accounts are now considerd alias accounts, so need to use same password. Yahoo server uses an app specific generated password. Gmail uses Oauth Authentication Method so if that is set up then it will not accept normal password unless trying to set up Oauth token. Xfinity needs you to logon to webmail account and select a checkbox to allow third party apps access.