Configuring Thunderbird SMTP Office 365 settings is essential if you want to send emails securely through Microsoft 365 using the Thunderbird email client. Incorrect SMTP settings can cause errors like “Outgoing server not working” or “Authentication failed.” In this detailed guide, you will learn how to properly configure Office 365 SMTP in Thunderbird and troubleshoot common issues.

Understanding Office 365 SMTP Requirements

Microsoft 365 uses modern authentication to enhance email security. Thunderbird supports secure SMTP connections through STARTTLS or SSL/TLS. Before configuring, ensure that your Microsoft 365 account allows SMTP AUTH.

Step 1: Open Thunderbird SMTP Settings

Open Thunderbird, click the Menu (☰), select Account Settings, and scroll down to Outgoing Server (SMTP). Choose the account you want to configure.

Step 2: Enter Office 365 SMTP Settings

Use the following SMTP details for Microsoft 365:

• Server Name: smtp.office365.com
• Port: 587
• Connection Security: STARTTLS
• Authentication Method: OAuth2 (recommended) or Normal Password
• User Name: Your full Microsoft 365 email

If STARTTLS fails, try port 465 (SSL/TLS) with encryption enabled.

Step 3: Ensure SMTP AUTH is Enabled in Office 365

SMTP AUTH must be enabled in your Microsoft 365 admin center. If it is disabled, Thunderbird will fail to send messages.

• Login to Microsoft 365 Admin Center.
• Go to Users → Active Users.
• Select your account and open Mail Settings.
• Enable the option Authenticated SMTP.

If you are not an admin, request your IT team to check or enable SMTP AUTH.

Step 4: Fix Thunderbird SMTP Authentication Issues

If Thunderbird cannot authenticate with Office 365, try these solutions:

• Make sure your Microsoft 365 password is correct.
• If MFA (Multi-Factor Authentication) is enabled, create an App Password.
• Ensure your email address is typed correctly in both incoming and outgoing settings.
• Disable firewall or antivirus software temporarily and test again.
• Update Thunderbird to the latest version for improved OAuth compatibility.

Step 5: Test Your Configuration

After saving your SMTP settings, send a test email. If the message is delivered successfully, your configuration is correct. If not, recheck your port numbers, security settings, and authentication method.

Why Thunderbird SMTP Office 365 Setup Fails?

Common reasons include incorrect port numbers, disabled SMTP AUTH, outdated Thunderbird version, or security blocks from Microsoft Defender. Ensuring accurate configuration prevents login failures and improves email performance.