Resolve Thunderbird sending problems quickly — SMTP errors, Outbox stuck, authentication failures, and other causes explained with easy fixes.
Get Immediate Help NowIf your outgoing mail is critical for business, our priority team can help restore sending functionality quickly with guided troubleshooting.
Connect with Support SpecialistOutgoing mail problems often stem from SMTP configuration, authentication, network restrictions or client-side issues. Below are the most common causes and quick tests you can perform.
Incorrect SMTP host, wrong port, or insecure connection settings will prevent sending.
Try: Verify SMTP host, port (465/587), and SSL/TLS settings with your provider.
Wrong password or blocked login attempts can cause SMTP authentication to fail.
Try: Re-enter credentials, enable OAuth2 if available, or reset your account password.
Firewalls, routers, or ISPs may block SMTP ports or throttle outgoing mail.
Try: Test sending on a different network or temporarily disable firewall/antivirus.
Follow these steps in order and try sending an email after each one to see if the problem is resolved.
Simple but effective — restart both Thunderbird and your PC to clear temporary glitches and network caches.
Go to Account Settings → Outgoing Server (SMTP). Ensure server name, port, security (SSL/TLS), and authentication method are correct.
Messages stuck in the Outbox can block sending. Delete or move stuck messages from Outbox and try again.
Log in to your email via webmail to confirm the account can send messages. If webmail also fails, the issue is with the provider.
Open Thunderbird in Troubleshoot Mode (Help → Troubleshoot Mode). If sending works, re-enable add-ons one-by-one to identify the conflict.
Temporarily disable security software to test if SMTP connections are blocked. If confirmed, add Thunderbird to the exception list.
If authentication errors persist, reset your email account password and reconfigure the account in Thunderbird using the correct credentials and authentication method (OAuth2 recommended).
Look at the error code shown by Thunderbird (e.g., SMTP 535 authentication failed or 421 service not available) and search provider documentation for exact meaning.
For business email, consider using your provider's authenticated relay or a third-party SMTP relay if ISP blocks default ports.
Backup your profile (Help → Troubleshooting Information → Profile Directory). Create a new profile to test whether the issue is profile-specific and selectively copy necessary files.
If you can't resolve the issue, our support specialists can help troubleshoot remotely and fix SMTP/authentication problems quickly.
Quick answers to common "Thunderbird email not sending" problems:
Authentication failed usually means wrong username/password or the provider requires app-specific password or OAuth2. Reset your password and update settings accordingly.
Delete or move the stuck messages, restart Thunderbird, and re-send. If the message is large, try removing attachments or using cloud links instead.
Yes — some ISPs block SMTP ports. Try using alternative ports (587 with STARTTLS) or authenticate via your provider's recommended relay.
Still having trouble? Describe your problem and attach any error messages or logs. Our team will respond with specific instructions.