After upgrading to Thunderbird 3, some Thunderbird users are getting an error message similar to "An error occurred sending mail: Unable to authenticate to SMTP server ..." when they send email.
This is caused by a bug in Thunderbird 2.0, where SMTP configuration settings that were previously ignored are now being used. Specifically, in Thunderbird 2, if an SMTP server did not require authentication, data in the user name / password fields was ignored. In Thunderbird 3, however, data in the user name and password fields is sent whether or not the server requires the data, causing some servers to reject the connection.
To fix this problem, modify the SMTP server settings. Select Tools | Account Settings and then Outgoing Server (SMTP) at the bottom of the account list. A list of the configured SMTP servers will be displayed in the right pane:
Edit the SMTP account that is reporting the error by selecting the account and pressing the Edit button. Uncheck the Use name and password box (in Thunderbird 3.0) or change the Authentication method to "No authentication" (in Thunderbird 3.1).