Fix Thunderbird Microsoft 365 POP Login Failure with OAuth2
Question details
Users are unable to connect to their Microsoft 365 email accounts via POP and SMTP in Thunderbird despite entering the correct credentials.
- Product
- Mozilla Thunderbird, Microsoft 365
- Device & OS
- not provided
- Scenario
- Checking or sending emails in Thunderbird using a Microsoft 365 account configured with POP/SMTP.
- Observed behavior
- Thunderbird suddenly prompts that the username is unknown or the password is bad, blocking email access.
Ensure you have your Microsoft 365 account email address and password handy. Also, verify that your Thunderbird application is updated to the latest version, as older versions may lack full OAuth2 support.
Switch Authentication Method to OAuth2
Update your server settings in Thunderbird to use OAuth2, which is now required by Microsoft for secure email access.
Microsoft has phased out Basic Authentication (Normal Password) for Exchange Online, requiring modern authentication (OAuth2) to improve security. Changing this setting allows Thunderbird to securely request access using a Microsoft login token instead of storing your password.
Launch Thunderbird, right-click on your Microsoft 365 email account in the left pane, and select Account Settings.
Navigate to Server Settings for your incoming POP account. Locate the Authentication method dropdown and change it from Normal Password to OAuth2.
Select Outgoing Server (SMTP) at the bottom of the left pane, click on your Microsoft 365 SMTP server, and select Edit. Change the Authentication method here to OAuth2 and click OK.
Restart Thunderbird or click Get Messages to prompt an email sync. A Microsoft 365 sign-in dialog will appear; log in with your account credentials and grant Thunderbird permissions if asked.
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Frequently Asked Questions
Why did my Thunderbird Microsoft 365 login suddenly stop working?
Microsoft recently disabled basic authentication (Normal Password) for most tenants to improve security. This requires modern authentication (OAuth2) for all email clients connecting via POP, IMAP, or SMTP.
What should I do if the Microsoft 365 OAuth2 sign-in window is blank in Thunderbird?
A blank sign-in window usually indicates blocked cookies or an outdated Thunderbird version. Update Thunderbird to the latest release and check your Privacy & Security settings to ensure cookies from login.microsoftonline.com are allowed.
Does changing to OAuth2 affect my existing emails in Thunderbird?
No, switching the authentication method only updates how Thunderbird verifies your identity with Microsoft's servers. Your locally stored emails and account data remain completely intact.
