Fix 3: Allow Less Secure Apps
Most email clients including the likes of Google and Yahoo flags third-party clients as less secure ones. However, since you are aware that Thunderbird is pretty much safe to use, you could allow less secure apps on Yahoo and Gmail. Follow the below steps for that:
For Gmail:

- Head over to the Security section of your Google Account.
- Scroll down until you reach the Less secure app access section.
- Then click on Turn on access (not recommended).
- Finally, enable the toggle next to Allow less secure apps.
For Yahoo:

- Go to Yahoo Mail’s Settings page
- Head over to the Account Security option from the left menu bar.
- Then scroll to the “Allow apps that use less secure sign-in” section.
- Now just enable the toggle next to the said option.
Once that is done on Google/Yahoo, launch the app, and see if it fixes the Thunderbird configuration could not be verified error.
Fix 4: Disable Two-Step Verification
While Two-Step Verification or Two Factor Authentication adds an extra layer of security, yet sometimes it might result in a few issues as well. So if your email account has this additional security enabled, then you could disable it for the time being and then try accessing Thunderbird. Given here are the steps to disable the authentication for Yahoo and Gmail:
For Gmail:

- Go to your Google Account’s Security section.
- Scroll to the Sign in to Google section and click on 2-Step Verification
- Now follow the on-screen instructions to disable this security measure.
For Yahoo:

- Head over to Yahoo Mail’s Settings page
- Then go to the Account Security option from the left menu bar.
- Now scroll to the Two-step verification section and disable the toggle.
Once you have disabled 2SV on your Google or Yahoo account, launch Thunderbird and check whether the configuration could not be verified error stands rectified or not.
Fix 5: Add Thunderbird to Firewall’s Whitelist
In some instances, your Firewall app might be a little over-protective and could raise a false positive against Thunderbird. As a result, the app will then not be able to function as expected. So the best bet, in this case, is to add the app to the Firewall’s whitelist. Here are the steps to do so on the Windows Firewall:
- Head over to the Start menu and search for Windows Defender Firewall.
- Then click on Allow an app or program through Windows Defender Firewall.

- Select the Thunderbird app from the list and click OK.
- However, if it isn’t present there, then click on the Change Settings button situated at the top right. You will need administrative privileges to carry out this task.

- Navigate to the Thunderbird app, select the file, and click OK.
Once this is done, restart your PC and see if the Thunderbird configuration could not be verified error has been fixed or not.
Fix 6: Enable Cookies
If you are using Mac, then it might be the case that your PC isn’t accepting cookies from Google. In that case, your Google ID might not be able to interact with Thunderbird. So we would recommend that you enable the cookies on your Mac setup. Here is how it could be done:

- On your Mac, go to the Safari option from the menu bar
- Then select Preferences from the drop-down menu.
- Finally, disable the Block all cookies option.
That’s it. These were the steps to fix the Thunderbird configuration could not be verified error. We have shared six different workarounds for the same, do let us know in the comments which method managed to rectify the issue. Likewise, here are some iPhone Tips and Tricks, PC tips and tricks, and Android Tips and Trick that deserve your attention as well.