Thunderbird users running Ubuntu 24.04 may encounter issues sending emails through Microsoft Outlook accounts due to compatibility or configuration problems. This issue typically manifests as emails getting stuck in the outbox or receiving error notifications when attempting to send messages. Fortunately, several effective troubleshooting methods can quickly restore Thunderbird's email sending functionality.

Method 1: Update OAuth2 Authentication Settings

Step 1: Open Thunderbird and navigate to the main menu. Click on Edit and select Account Settings from the dropdown menu.

Step 2: In the Account Settings window, locate your Microsoft Outlook account on the left panel and click on Server Settings.

Step 3: Under the Server Settings section, ensure the Authentication method is set to OAuth2. OAuth2 authentication is required by Microsoft for secure connections, and outdated methods can cause sending failures.

Step 4: Next, navigate to the Outgoing Server (SMTP) section at the bottom of the left panel. Select your Outlook SMTP server and click on the Edit button.

Step 5: In the SMTP Server settings window, verify that the Authentication method is also set to OAuth2. Confirm the server details match the official Microsoft SMTP settings:

  • Server Name: smtp.office365.com
  • Port: 587
  • Connection Security: STARTTLS
  • Authentication Method: OAuth2

Step 6: Save your changes by clicking OK. Restart Thunderbird to apply the updated settings. You will be prompted to log in again through Microsoft's OAuth2 login window—complete this step to authorize Thunderbird properly.

Once completed, attempt to send an email to verify that the issue is resolved.

Method 2: Reauthorize Thunderbird's Access to Outlook Account

Sometimes, authorization tokens can expire or become corrupted, blocking Thunderbird from sending emails. Reauthorizing your Outlook account within Thunderbird can quickly resolve this.

Step 1: Open Thunderbird, click on the hamburger menu (three horizontal lines) at the top-right corner, and select Preferences.

Step 2: Navigate to the Privacy & Security tab, scroll down to the section named Passwords, and click on Saved Passwords.

Step 3: Locate and select all entries related to your Outlook account, then click on the Remove button. Confirm the deletion when prompted.

Step 4: Close Thunderbird completely, then reopen the application. Thunderbird will prompt you to re-enter your Outlook account credentials through Microsoft's OAuth2 authentication window. Enter your login details and grant Thunderbird the necessary permissions.

After successful reauthorization, test sending an email again to confirm the fix.

Method 3: Update Thunderbird and Ubuntu Packages

Compatibility issues between Thunderbird and Ubuntu 24.04 can sometimes trigger email sending errors. Ensuring your software is fully updated can resolve these conflicts.

Step 1: Open the terminal by pressing Ctrl + Alt + T.

Step 2: Enter the following commands one by one to update your system packages and Thunderbird:

sudo apt update

Step 3: After the updates complete, restart Thunderbird to ensure the latest version and libraries are loaded correctly.

Attempt sending an email to verify that the issue has been resolved.

Method 4: Check Firewall and Antivirus Settings

Occasionally, firewall or antivirus software might inadvertently block Thunderbird's outgoing email ports, causing sending issues.

Step 1: Open your firewall settings or application (such as UFW or GUFW) and verify that outgoing connections on port 587 (SMTP) are allowed.

Step 2: If you use antivirus software, check its settings to ensure it is not blocking Thunderbird or outgoing SMTP connections. Temporarily disabling antivirus software can help identify if it is the cause.

Once firewall and antivirus configurations are adjusted, retry sending an email in Thunderbird.

After following these steps, your Thunderbird application on Ubuntu 24.04 should successfully send emails through your Microsoft Outlook account. Regularly checking for updates and ensuring OAuth2 authentication remains active will help prevent similar issues in the future.