If you are facing the issue where Thunderbird cannot connect to Office 365, you are not alone. Many users experience connection failures when trying to access their Microsoft 365 email accounts using Mozilla Thunderbird. The problem commonly arises due to authentication changes made by Microsoft, incorrect IMAP/SMTP settings, outdated security configurations, or missing OAuth2 support. This detailed 800-word guide explains why the issue occurs and how to fix it step-by-step.
The main reason Thunderbird fails to connect with Office 365 is because Microsoft has officially disabled Basic Authentication for email access. Office 365 now requires OAuth2, a modern and secure authentication method. If your Thunderbird version is outdated or still using Basic Authentication, it will fail to log in. Additionally, incorrect IMAP and SMTP server settings or disabled modern authentication in the Microsoft admin center can also cause connection issues.
Before modifying any settings, ensure that you are using the latest version of Thunderbird. Only updated versions support OAuth2 authentication. Using an older version creates login errors because Office 365 no longer accepts outdated login protocols. Go to the Thunderbird menu, select “Help,” and choose “About Thunderbird” to check and update your software.
When Thunderbird cannot connect, verify that your IMAP settings are correct. Office 365 requires very specific server configurations:
Incoming Mail (IMAP)
Server: outlook.office365.com
Port: 993
Security: SSL/TLS
Authentication: OAuth2
If your authentication is still using “Normal Password,” you must switch it to OAuth2. This is essential for establishing a secure connection.
Sending emails through Thunderbird also requires accurate SMTP configuration. Incorrect outgoing mail settings may cause errors like “Login Failed” or “Unable to Connect.” Use the following details:
Outgoing Mail (SMTP)
Server: smtp.office365.com
Port: 587
Security: STARTTLS
Authentication: OAuth2
After updating these fields, save your settings and restart Thunderbird. You will be prompted to log in using the Microsoft OAuth2 login window.
If OAuth2 is still not appearing in Thunderbird, your Office 365 admin might have disabled Modern Authentication. Without it, IMAP and SMTP connections will fail. Log into the Microsoft admin center and ensure that “Modern Authentication” is enabled. This setting allows third-party apps like Thunderbird to connect securely.
Some organizations use strict conditional access policies. These policies may block Thunderbird if it does not meet required security standards. Make sure your device meets all compliance rules. If you're a home user, these restrictions are usually not applied; however, business or school accounts may require admin approval to allow IMAP connections.
If Thunderbird repeatedly asks for your password or rejects the login, follow these steps:
• Remove saved passwords from Thunderbird and try again.
• Re-authenticate using the Microsoft login popup.
• Ensure two-factor authentication is enabled for your Microsoft account.
• Generate an app password only if the admin requires it (rare case).
If the OAuth2 popup does not appear, delete the email account from Thunderbird and re-add it using the automated setup. Thunderbird will automatically detect Office 365 settings and use OAuth2 if supported. This often resolves hidden configuration glitches.
Why is Thunderbird not accepting my Office 365 password?
Because Office 365 now requires OAuth2, not basic passwords. Update your authentication method.
Can I still use IMAP with Office 365?
Yes, but only if OAuth2 and Modern Authentication are enabled.
Does Thunderbird support Office 365?
Yes, Thunderbirds latest versions fully support Office 365 with OAuth2 login.
Why am I getting the error “Login Failed”?
Incorrect authentication or outdated Thunderbird software is usually the cause.