How to fix the Thunderbird “login to server failed” error
This means Thunderbird could not sign in to your incoming mail server. It is usually a stale saved password, a changed password, or the wrong authentication method for providers like Gmail. Jump to your situation below or work through the methods in order.
By Neeraj Singh ~6 min Updated Jun 2026 88% found this helpful
Error message
Login to server failed. The username or password is incorrect, or sign-in was rejected.
Summary
The login to server failed message means Thunderbird could not authenticate with your incoming mail server. The most common causes are a stale saved password (you changed the password elsewhere but Thunderbird still has the old one), or the wrong authentication method. Major providers such as Gmail and Yahoo no longer accept your normal password from third-party apps, they require OAuth2 or an app password, so a normal-password setup fails to log in. The fix is to clear the saved password in Settings, Privacy and Security, Saved Passwords, then restart Thunderbird so it prompts for a fresh sign-in. For Gmail and Yahoo, set the authentication method to OAuth2 (which opens the provider's own sign-in), or generate an app password if you use two-factor authentication. Confirming the username is your full email address completes the fix.
What this error means
Receiving mail requires Thunderbird to log in to the incoming IMAP or POP server. When that login is rejected, you get login to server failed. The credentials Thunderbird is sending, or the way it is sending them, are not accepted by the server.
In recent years the biggest cause has shifted to authentication method. Google, Yahoo and others turned off plain-password access for third-party apps, so Thunderbird must use OAuth2 or an app password. Combined with the classic stale-saved-password problem, those cover almost every case.
Common causes
Thunderbird is using a stale or wrong saved password.
The account password was changed elsewhere.
The provider requires OAuth2, not a normal password.
Two-factor authentication requires an app password.
The username is not the full email address.
The wrong authentication method is selected.
Expert insight
“Login failed used to be a simple wrong password, and sometimes it still is, but these days it is usually the provider changing the rules. Gmail and Yahoo stopped letting apps sign in with your normal password, so Thunderbird has to use OAuth2 or an app password. My routine is to delete the saved password so Thunderbird stops trying the old one, restart so it asks fresh, and switch Gmail and Yahoo accounts to OAuth2. That clears the vast majority of these.”
Manager, Tech Support & Operations · 19+ years fixing Windows and system errors
✓ How to fix it
Method 1
Delete the saved password
1Go to Settings, Privacy and Security, Saved Passwords, and remove the entry for the failing account.
2Restart Thunderbird so it prompts for the password again.
3Enter the current password fresh.
Method 2
Use OAuth2 for Gmail and Yahoo
1In Account Settings, set the incoming server's Authentication method to OAuth2.
2Thunderbird then opens the provider's own sign-in page to authorise access.
3This is required for Gmail and Yahoo.
Method 3
Use an app password with two-factor
1If the account has two-factor authentication, generate an app password in the provider's security settings.
2Use that app password in Thunderbird instead of your normal one.
3This is needed when OAuth2 is not available.
Method 4
Confirm the username
1Make sure the username is your full email address.
2A short username is a common cause of a failed login.
3Correct it and retry.
Method 5
Check the authentication method
1If not using OAuth2, set the method to Normal password for providers that accept it.
2Match the method to what your provider supports.
3Save and sign in.
Login failed is an incoming-server sign-in problem, so start by deleting the saved password and letting Thunderbird re-prompt, which fixes the classic stale-password case. For Gmail and Yahoo, the real fix is usually OAuth2 (or an app password with two-factor), since they no longer accept a normal password from third-party apps. Always use your full email address as the username.
Frequently asked questions
Why does Thunderbird say login to server failed?
It could not authenticate with your incoming mail server. Usually a stale saved password, a changed password, or the wrong authentication method, such as a normal password where the provider now requires OAuth2.
How do I clear a saved password in Thunderbird?
Go to Settings, Privacy and Security, Saved Passwords, remove the entry for the account, then restart Thunderbird so it prompts for the current password.
Why won't my Gmail password work?
Google no longer lets third-party apps sign in with your normal password. Set the authentication method to OAuth2, or use an app password if you have two-factor authentication enabled.
What is an app password?
A provider-generated password for a specific app, used instead of your normal password when two-factor authentication is on. Generate one in the account's security settings and use it in Thunderbird.
Should my username be the full email?
Yes. Most providers require the full email address as the username. A short username is a common reason a login fails.
What is OAuth2 and how do I enable it?
OAuth2 lets Thunderbird sign in through the provider's own page without storing your password. Set the incoming server's authentication method to OAuth2 in Account Settings, and a sign-in window appears.
Still not working?
If a fresh password and OAuth2 still fail, the provider may be blocking the sign-in as coming from an unrecognised app or location. Check the account's security or recent-activity page in webmail, approve the Thunderbird sign-in, and ensure access for mail apps is enabled. You can also submit your error to us for a tailored fix.
