How to fix Thunderbird SMTP error 5.5.1 (authentication required)
This means the outgoing (SMTP) server wants you to log in, but authentication is switched off in Thunderbird's outgoing server settings. Turning it on resolves it. Jump to your situation below or work through the methods in order.
By Neeraj Singh ~5 min Updated Jun 2026 90% found this helpful
Error message
5.5.1 Authentication Required. The server requires you to sign in before sending.
Summary
Error 5.5.1 means the outgoing (SMTP) server requires authentication, you must sign in to send, but Thunderbird's outgoing server is configured with authentication turned off. Almost every modern provider requires SMTP authentication, so a server set to No authentication in Thunderbird is refused with 5.5.1. The fix is in Account Settings, Outgoing Server (SMTP): change the authentication method from None to Normal password (or OAuth2 for Gmail and Yahoo), enter your username (your full email address), and use a submission port such as 587 with STARTTLS or 465 with SSL/TLS. Once Thunderbird is set to authenticate, the server accepts the connection and the message sends.
What this error means
Sending mail through an SMTP server almost always requires you to prove who you are first. If Thunderbird's outgoing server is set to use no authentication, it connects without logging in, and the server rejects the message with 5.5.1, authentication required.
So unlike 5.7.1, which is usually a wrong username or password, 5.5.1 is the login step being switched off entirely. Turning authentication on, and giving Thunderbird the username and method to use, is all that is needed.
Common causes
The outgoing server authentication method is set to None.
Use name and password is not enabled for the SMTP server.
No username is configured for the outgoing server.
The wrong authentication method is selected.
A non-submission port is used that requires no auth but is blocked.
Expert insight
“5.5.1 is the simplest one to fix because it is just a switch in the wrong position. The outgoing server wants a login and Thunderbird is set to not bother. I open the SMTP settings, change Authentication method from None to Normal password, put the full email address in as the username, and make sure it is on port 587 or 465. That is it, the server stops complaining and the mail sends.”
Manager, Tech Support & Operations · 19+ years fixing Windows and system errors
✓ How to fix it
Method 1
Turn on authentication
1Open Account Settings, Outgoing Server (SMTP), select the server and click Edit.
2Change Authentication method from None to Normal password (or tick Use name and password).
3This tells Thunderbird to sign in before sending.
Method 2
Pick the right method
1Use Normal password for most providers.
2Use OAuth2 for Gmail and Yahoo accounts.
3Save the change.
Method 3
Enter the username
1Set the User Name to your full email address.
2Thunderbird will prompt for the password on the next send.
3Use an app password where the provider requires one.
Method 4
Use a submission port
1Set the port to 587 with STARTTLS, or 465 with SSL/TLS.
2These submission ports expect authentication.
3Match Connection security to the port.
Method 5
Send a test
1Send a test message to confirm the server now accepts the authenticated connection.
2If it fails with 5.7.1 instead, the method worked but the username or password is wrong.
5.5.1 is authentication being switched off, so the fix is simply to turn it on: set the outgoing server's authentication method to Normal password (or OAuth2 for Gmail and Yahoo) and give it your full email address as the username. Use a submission port, 587 with STARTTLS or 465 with SSL/TLS, which expect an authenticated connection.
Frequently asked questions
What does Thunderbird error 5.5.1 mean?
It means the outgoing (SMTP) server requires you to log in before sending, but Thunderbird's outgoing server is set to use no authentication, so the server refuses the message.
How do I turn on SMTP authentication in Thunderbird?
In Account Settings, Outgoing Server (SMTP), edit the server and change Authentication method from None to Normal password, then enter your full email address as the username.
What is the difference between 5.5.1 and 5.7.1?
5.5.1 means authentication is turned off entirely, so the server refuses you. 5.7.1 means authentication is on but your credentials or From address were rejected.
Which port should I use?
Use 587 with STARTTLS or 465 with SSL/TLS. These submission ports are designed for authenticated sending and are what most providers expect.
Should I use Normal password or OAuth2?
Use Normal password for most providers. Use OAuth2 for Gmail and Yahoo, which require it. Either way, enter your full email address as the username.
It now says 5.7.1, what changed?
Authentication is now on, which is progress. A 5.7.1 means the username or password is wrong, so correct those in the outgoing server settings.
Still not working?
If authentication is on with the correct method and it still fails, the provider may require OAuth2 specifically, or an app password because two-factor authentication is enabled. Switch the method to OAuth2, or generate an app password in the account's security settings and use that. You can also submit your error to us for a tailored fix.
