How to Fix Thunderbird SMTP Login Failure with Office 365
Question details
The user is unable to send emails via Thunderbird using an Office 365 account due to SMTP login failures.
- Product
- Mozilla Thunderbird / Office 365
- Device & OS
- not provided
- Scenario
- Sending emails through smtp.office365.com using port 587, STARTTLS, and OAuth2 authentication.
- Observed behavior
- Thunderbird fails to authenticate and send mail despite having the correct server and OAuth2 configurations set up.
Ensure you have your correct Office 365 account credentials and that your organization's IT policy allows third-party email clients. Verify that your internet connection is stable and not blocking SMTP port 587.
Verify Server Settings and OAuth2 Authentication
Double-check that your SMTP settings exactly match Microsoft's requirements for Office 365 and force a new OAuth2 token prompt.
Office 365 requires strict adherence to its security protocols. Even a minor misconfiguration in the outgoing server settings can cause the login to fail. Resetting the OAuth2 token can also resolve hidden authentication glitches.
In Thunderbird, go to Tools > Account Settings and select Outgoing Server (SMTP) at the bottom of the left pane.
Select your Office 365 SMTP server and click Edit. Ensure the server name is strictly smtp.office365.com and the port is set to 587.
Set Connection security to STARTTLS and the Authentication method to OAuth2. Ensure your username is your full email address.
Go to Settings > Privacy & Security > Saved Passwords. Search for your Office 365 SMTP entries and remove them to force a fresh login prompt on your next send attempt.
Contact Thunderbird Support
If settings are correct and the issue persists, the problem may require account-specific investigation by the support team.
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Frequently Asked Questions
Why does Office 365 block Thunderbird SMTP?
Microsoft periodically updates its security policies. If your organization disables 'Authenticated SMTP' or enforces strict modern authentication policies, Thunderbird might be blocked unless OAuth2 is properly configured and Authenticated SMTP is enabled in the Microsoft 365 admin center.
Does Thunderbird fully support OAuth2 for Office 365?
Yes, modern versions of Thunderbird fully support OAuth2 for Microsoft 365 accounts. Ensure you are running the latest version of Thunderbird and have selected 'OAuth2' as your authentication method in both your IMAP and SMTP settings.
How do I enable Authenticated SMTP in Office 365?
An IT administrator must log into the Microsoft 365 Admin Center, go to Active users, select the specific user account, click on the Mail tab, and check 'Manage email apps' to ensure that the 'Authenticated SMTP' option is enabled.
