Fix Thunderbird Cannot Verify Microsoft 365 IMAP Settings
Question details
Users are unable to sync their Microsoft 365 email accounts via IMAP in Thunderbird because valid passwords are continuously rejected.
- Product
- Mozilla Thunderbird and Microsoft 365
- Device & OS
- not provided
- Scenario
- Setting up or authenticating a Microsoft 365 email account in Thunderbird using IMAP.
- Observed behavior
- Thunderbird repeatedly rejects the correct Microsoft 365 account password and fails to verify IMAP settings because basic authentication is disabled by Microsoft.
Ensure you have your Microsoft 365 login credentials available and check if your organization enforces Multi-Factor Authentication (MFA).
Change Authentication Method to OAuth2
Switch Thunderbird's authentication protocol to OAuth2, which is required for modern Microsoft 365 authentication.
Microsoft has deprecated basic authentication (normal passwords) for Microsoft 365 IMAP connections to enhance security. Upgrading your authentication method to OAuth2 prompts a secure Microsoft sign-in window to grant Thunderbird access.
Launch Thunderbird, right-click on your Microsoft 365 email address in the left pane, and select 'Settings'.
Navigate to 'Server Settings' under your account. Locate the 'Authentication method' dropdown and change it from 'Normal password' to 'OAuth2'.
Scroll down the left pane and click on 'Outgoing Server (SMTP)'. Select your Microsoft 365 server, click 'Edit', and change the Authentication method to 'OAuth2'.
Restart Thunderbird. A Microsoft sign-in window will pop up. Enter your credentials, complete any MFA prompts, and grant Thunderbird permission to access your account.
Generate and Use an App Password
Create a unique app password for Thunderbird if your organization's setup prevents OAuth2 from functioning correctly.
Enable IMAP and Authenticated SMTP (Admin Only)
Administrators must verify that IMAP and Authenticated SMTP protocols are enabled for the affected user's account.
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Frequently Asked Questions
Why is Thunderbird suddenly rejecting my Microsoft 365 password?
Microsoft has phased out Basic Authentication (normal passwords) for Exchange and IMAP connections. To resolve this, you must switch your authentication method in Thunderbird to OAuth2, which utilizes modern, secure token-based authentication.
What is OAuth2 in Thunderbird?
OAuth2 is a modern authentication standard that allows Thunderbird to access your Microsoft 365 account securely. Instead of storing your password directly, Thunderbird uses a secure token generated after you log in through the official Microsoft sign-in window.
How do I know if my Microsoft 365 IMAP access is disabled?
If you are an end user, you may receive continuous connection timeouts or permission errors even with OAuth2 enabled. You will need to contact your IT administrator to verify that IMAP and Authenticated SMTP are turned on for your specific account in the Microsoft 365 Admin Center.
Can I still use a normal password if I don't want to use OAuth2?
Directly using a normal password is no longer supported by Microsoft 365 for IMAP. However, if your organization allows it, you can enable Two-Step Verification on your Microsoft account, generate a dedicated 'App Password', and use that in Thunderbird instead.
