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Thunderbird not sending emails can be quite frustrating, especially when you’re in a hurry and need to send an important message. This Thunderbird SMTP error is a common issue that many users face. But no worries, for there are various solutions to fix this problem and send messages with ease. Here, we’ll explore the best solutions to tackle the Thunderbird SMTP error and get your emails delivered successfully.

SMTP Error: Message Cannot Be Sent

Sometimes while using Mozilla Thunderbird users have encountered issues in sending messages. SMTP server error or Thunderbird email not responding is the main reason behind this problem.

To send and receive an email, SMTP, an internet standard, is used by mail servers. With the help of the SMTP error message, one can understand the problem behind the message wasn’t sent successfully. And one can check the bounce error message to diagnose the bouncing incoming and outgoing message problem.

Solutions to fix Thunderbird SMTP Server Error: Message Cannot Be Sent

Once in a while, the issue is fine, but what if you’re constantly facing Thunderbird issues? it is high time you check out for solutions.

Solution 1 Check IMAP and SMTP Settings

While using the Thunderbird email client, there are some issues faced by users. To solve the issue, follow the steps below-

  • Log in to your Mozilla Thunderbird application.
  • Go to Tools and select Account settings.
  • Click on the Outgoing Server (SMTP) on the window that appears.
  • Check whether your mail server in the setting detail is the same as recommended by Mozilla.

Start the search for SMTP mail settings, if the user’s mail setting couldn’t be found.

Follow the steps given below to check your SMTP.

  • Open the application and select Account Settings.
  • Go through your email address and check your username and password.
  • Always try to make sure of using the correct SMTP server for the email address. using the correct SMTP server

By following the above steps, you will be able to check whether you are using the correct SMTP.

Solution 2- Check outgoing email is not being blocked

In this solution, one has to verify that your outgoing email is not being blocked by your firewall, antivirus, or your Internet Service Provider. For this, follow the given steps-

  • Try sending a test email turning both antivirus and firewall software off. Then turn them on and send another test email. By this, you can verify the cause of the error.
  • Many ISPs block outgoing emails on port 25. You may have to switch to another port like 587 or 465 and contact your ISP’s support to see if they are blocking your outgoing email port.
Solution 3- Update Mozilla Thunderbird

For TLS network encryption protocol, Thunderbird 78 is looking for a higher security standard. In case the newer version of the protocols is not supported by the provider, sending and receiving messages may fail. So update the Mozilla Thunderbird application to the latest version.

  • From the Help menu, tap on About Thunderbird.
  • Thunderbird will start to check for updates and download them.check for Thunderbird updates and download
  • As the updates are all set to be installed, tap on Restart Thunderbird to Update.Restart Thunderbird to Update
Solution 4- Delete and Reset SMTP Password

Try deleting your SMTP password.
Note: However, don’t do this as a first step, especially if you cannot send messages after updating Thunderbird. In such a case, usually, there is nothing wrong with a stored SMTP password in Thunderbird.

  1. Access Thunderbird and go to the tab Privacy & Security.Access Thunderbird
  2. Under the Passwords section, click on Saved Passwords.Under the Passwords section
  3. Choose the SMTP one and tap on Remove.Choose the SMTP

Now configure the email account and try to send an email again; this can resolve the Thunderbird email not sending issue.

Otherwise, Just change your SMTP password by using a ‘reset email password’ from your ISP’s support website.

Solution 5: Migrate to Another Email Client

If the above solution list doesn’t help, we will highly recommend Kernel for MBOX to PST. This helps users to switch to another email client like Outlook for a better emailing experience.

Outlook helps to manage all your accounts from one application without signing up for a new array of services and new accounts. It can easily connect all your email accounts. It implies that one can search, manage emails, and apply other email features across all accounts without using any complicated filters.

The tool easily runs on all MS Outlook versions (2019, 2016, 2013, 2010, 2007, and 2003). This also moves your total emails of Apple Mail to Outlook PST files with complete accuracy, retaining the original formatting, structure, and properties.

If you want more of such powerful professional tools, you can checkout our Kernel Website for more such certified tools, which supports backup apple mail data to iCloud and a lot more.


It can be annoying to MBOX users when the Message Cannot Send issue persists. Even though this issue can be easily fixed, some users may like to switch to Outlook. One can switch to Outlook from Thunderbird using the MBOX to Outlook PST tool. It supports multiple MBOX files from various clients for conversion and easily converts all emails and attachments to PST, DBX, MSG, and EML and it also works over export thunderbird emails to Gmail without causing any disturbance to the original Meta properties. If you need some more Thunderbird troubleshooting assistance, then stay tuned for our upcoming troubleshoot articles.

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