The correct SMTP (outgoing) server setting should be:

SMTP server name: smtp-mail.outlook.com (Note: It is not smtp.office365.com)

SMTP port: 587 (Your setting is correct.)

SMTP encryption: STARTTLS (You might need to check this setting, too.)

Authentication Method: OAuth2/Modern Auth (Your setting is correct.)

Reference: https://support.mozilla.org/en-US/questions/1473104#answer-1712116

It sounds like you're encountering an issue with sending emails using your Outlook account in Thunderbird. You've already tried several good troubleshooting steps, but there are still some additional things to check and try. Possible Causes and Solutions 1. Ensure Correct SMTP Settings (Office 365)

Double-check that the SMTP settings for Office 365 (Outlook) are correct. These are the typical SMTP settings for Office 365:

   SMTP Server: smtp.office365.com
   Port: 587
   Encryption: STARTTLS (or TLS depending on the version of Thunderbird)
   Authentication Method: OAuth2 (which you've correctly set)
   Username: Your full Outlook email address (e.g., yourname@outlook.com)

Since you mentioned that you are using OAuth2, make sure OAuth2 is selected for the authentication method in Thunderbird for the Outgoing Server (SMTP) settings.

Steps to verify the SMTP settings:

   Go to Tools > Account Settings.
   In the left sidebar, under Outgoing Server (SMTP), select the SMTP server used by your Outlook account (usually smtp.office365.com).
   Click Edit, and check that the settings are exactly as above.
   Ensure that OAuth2 is selected as the Authentication method, and that the username is your full Outlook email address.

2. Remove and Reconfigure OAuth2 Authentication

If the authentication with OAuth2 is still failing, it could be an issue with the cached credentials. You may need to reset your OAuth2 settings:

Steps to remove OAuth2 credentials:

   Go to Tools > Options (or Preferences on macOS).
   Click on Privacy & Security.
   Scroll down to Saved Logins and click View Saved Logins.
   Delete any saved logins related to your Outlook account.
   Reconfigure OAuth2: Go back to Account Settings and make sure the SMTP server is set to use OAuth2 for authentication. If needed, re-enter your Outlook email password and let Thunderbird prompt you for OAuth2 login again.

3. Check Two-Factor Authentication (2FA)

If your Outlook account has two-factor authentication (2FA) enabled, make sure that Thunderbird is properly handling the OAuth2 flow. Sometimes, 2FA can interfere with Thunderbird's ability to authenticate properly, requiring you to log in through the OAuth2 dialog every time you try to send an email.

Steps to check if 2FA is affecting the login:

   Log into your Outlook account in a browser.
   Check if you have any unusual activity or security settings in your Microsoft account.
   If possible, generate an App Password for Outlook to see if it resolves the issue (especially if you're trying to use basic authentication or OAuth2 issues).

4. Test Basic Authentication (as a temporary test)

You mentioned trying basic authentication for SMTP, but it seems like the issue persists. Let's test it again with the correct settings, just to rule out OAuth2 being the cause.

Steps to switch to Basic Authentication:

   Go to Account Settings > Outgoing Server (SMTP).
   Select your SMTP server (smtp.office365.com).
   Change the Authentication method to Normal password instead of OAuth2.
   Enter your full Outlook email address as the username and the App password (if 2FA is enabled) or your regular password for your Outlook account.
   Test sending an email again. If this works, the issue is likely with the OAuth2 configuration.

5. Check for Firewall or Antivirus Interference

Your firewall or antivirus software could be blocking the connection to the SMTP server. Try disabling your antivirus or firewall temporarily and see if that resolves the issue. If it works, make sure to add Thunderbird as an exception in your firewall/antivirus settings. 6. Check for Thunderbird Updates

If you are using an outdated version of Thunderbird, it could be causing compatibility issues with OAuth2 or Office 365. Ensure you're running the latest version of Thunderbird.

Steps to check for updates:

   Go to Help > About Thunderbird.
   If there is an update available, it will be downloaded and installed automatically.

7. Review OAuth2 Errors

Sometimes, Thunderbird can give you more details about the error when it tries to authenticate with the SMTP server.

   Go to Help > Troubleshooting Information.
   Look for any OAuth2 related errors or logs in the Logins section or the Errors section.

This could give you more specific information about what’s going wrong with the authentication. Summary of Key Steps to Try:

   Verify SMTP Settings: Double-check server, port, and authentication methods.
   Clear OAuth2 Credentials: Remove saved logins and reconfigure OAuth2.
   Check Two-Factor Authentication: If enabled, ensure Thunderbird can handle OAuth2 properly.
   Try Basic Authentication Temporarily: Use an app password or Outlook password for SMTP.
   Check Firewall/Antivirus: Ensure no software is blocking the connection.
   Update Thunderbird: Make sure you're using the latest version of Thunderbird.
   Look for OAuth2 errors in Troubleshooting Information.

Hopefully, these steps will help resolve your issue. Let me know if you continue to experience problems, and I can assist further!