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How to Fix Thunderbird Not Sending Emails? Complete Guide

By Admin

Updated: March 30, 2026 ~ 5 Minutes Reading

Table of Contents

Is your Mozilla Thunderbird not sending emails? i.e., when you try to send emails, Thunderbird freezes, shows an SMTP error, or recipients aren’t getting them. Here we have listed common causes why Thunderbird outgoing mail is not working and tested troubleshooting steps to fix the issue.

User Query from Thunderbird forum asking about why their Thunderbird is not able to send emails.

Reasons: Why Thunderbird Failed to Send Emails?

Incorrect SMTP settings (server address, port number, authentication method, password) are the most common cause why Thunderbird won’t send emails:

  • Thunderbird tries to send email using Port 25 (no longer supported by many internet service providers).
  • Using outdated encryption instead of SSL/TLS, STARTTLS.
  • Email services are shifting from older app passwords to OAuth2 for better security.

There can be other reasons, such as:

  • Your mailbox storage quota is exceeded – delete unnecessary emails
  • Unstable internet connection – switch to a better network
  • Outdated Thunderbird version – Update Thunderbird
  • Firewall, antivirus, and proxy interference

Important Checks Before Trying to Troubleshoot

Mozilla Thunderbird won’t send emails in the following cases:

  • Attachment size is greater than the set limit by ISP (generally 25 MB)
  • Sender’s address is incorrect and invalid
  • Your email address is blocked by the sender.

Step-by-Step Troubleshooting – Thunderbird Not Sending Emails

If you have validated the above reasons and tried out generic fixes like restarting, switching internet connection, and your cloud storage and mailbox are not full.

The fixes given below will help you resolve Thunderbird mail sending errors on Mac, Windows, or Linux devices. Whether you are using any email service like Gmail, Office 365, Yahoo, or any IMAP/POP account in Thunderbird.

Fix 1: Verify Thunderbird SMTP Settings

  1. Open Thunderbird on your computer.
  2. Click on Menu (three lines) >> Account Settings
  3. Scroll to the bottom of the page and go to Outgoing Server.
  4. Verify and enter the correct settings: Outgoing Server Name, Port, Connection Security, and Authentication Method.
  5. Press Ok and then Restart Thunderbird.
    Thunderbird SMTP Settings
Service Outgoing Server (SMTP) Port Connection Security Authentication Method
Gmail smtp.gmail.com 465 / 587 SSL/TLS (465) or STARTTLS (587) OAuth2
Outlook.com smtp.office365.com 587 STARTTLS OAuth2
Office 365 smtp.office365.com 587 STARTTLS OAuth2
Yahoo Mail smtp.mail.yahoo.com 465 / 587 SSL/TLS or STARTTLS OAuth2 / App Password
iCloud Mail smtp.mail.me.com 587 STARTTLS App Password

Fix 2: Edit Outgoing Description

If you have multiple email accounts of the same server (like two different Gmail accounts), Thunderbird may mix up entries stored associated with each account.

This leads to Thunderbird mail sending error, i.e., the server rejects the sender. To fix this:

  1. Go to Account Settings
  2. Scroll to Outgoing Server Settings >> Enter Configuration
  3. Give a unique description to each of the accounts.
  4. Save changes and try resending emails

Fix 3: Thunderbird Not Sending Emails Due to Old Password

When you try to send emails in Thunderbird, and you get an error like “Thunderbird cannot connect to the server” or “Keeps asking for password,” an outdated saved password is the issue.

  1. Go to Settings >> Security & Passwords >> Saved Passwords and change the password in Thunderbird.
  2. Or simply enter the new password when prompted. Check the Remember Me box when updating the password.

Fix 4: Thunderbird Won’t Send Email to Group

Note: Only try out this fix if you are able to send individual emails however with group or multiple emails you see “Timeout” or it simply freezes or takes way too long.

  1. Open Mozilla Thunderbird on your PC.
  2. Click on the Gear Icon (Settings) at the bottom.
  3. Search for and open the Config Editor settings
  4. Scroll to dom.max_script_run_time
    Thunderbird Failed to Send Emails due to config editor timed out fix
  5. Click on Edit and increase the value.
  6. Save changes and restart Thunderbird

Fix 5: Update to a newer version of Thunderbird

Older versions (before version 78) of Thunderbird are not compatible with modern OAuth, causing Thunderbird to fail to send an email.

  1. Click on Menu
  2. Select Help >> About Thunderbird
  3. Press Restart to update Thunderbird to reinstall Thunderbird.
    update & restart Thunderbird

FAQ: Thunderbird Can’t Send Email

Q1: Why am I not able to send emails in Thunderbird, as the Send button is greyed out?
If the Thunderbird Send or New Mail button is not working, it is likely due to an add-on interference. Go to Menu >> Troubleshooting Info >> Restart Thunderbird in Safe Mode.

Q2: Why is my Thunderbird message stuck in the outbox?
Thunderbird may not send emails from the Outbox because of ISP storage quota exceeded issues, incorrect SMTP settings, antivirus or firewall interference, or unstable internet connection.

Q3: What does “550 5.1.0 Authentication Required” mean in Thunderbird?
This error means the SMTP server is rejecting your email because the proper login credentials were incorrect or not valid, thus Thunderbird is looking for the correct validation method.

Still Facing Thunderbird Not Sending Emails Issue?

If your communication gets repeatedly hampered by the Thunderbird outgoing mail not working issue, get the professional Thunderbird Migration Tool to migrate Thunderbird to Outlook, Office 365, Gmail, and other email clients with a similar look and feel while maintaining complete data integrity.

Select the Live Chat button at the top of the page to get in touch with our Thunderbird experts.