Thunderbird connection to server was reset: Avast Mail Shield blocks Thunderbird – Avast Internet Security (Premium version) includes an email protection module that is known to be verified by Thunderbird. In this case, the only thing you can do to fix the problem is to disable the Mail Shield feature in the Core Shield in the Avast settings menu.
A modified version of Thunderbird. According to some affected users, this issue may also occur if you are using a modified version of Thunderbird. In this case, it is possible that your own Windows firewall threatens the executable file as a potential security risk and blocks it. To prevent this, you must whitewash the executable file in the Windows
Firewall settings menu.
AVG Shield blocks Thunderbird. Another module that has been reported to cause problems with Thunderbird is the email protection feature in AVG Antivirus. As with Avast Mail Shield, this security feature is enabled by default, so the only way to resolve this conflict is to disable it in the AVG Antivirus Settings menu.
Overprotective AV suite. In some cases, the reason Thunderbird throws this error is that the main executable is blocked by an overly secure package or firewall. If this scenario applies, you may be able to resolve the issue by whitelisting the Thunderbird executable or by completely removing the excessively protected package. Thunderbird connection to server was reset
Disable Mail Shield with Avast (if applicable)
One of the most common causes of this problem is the Avast mail scanning function called Mail Shield. This security feature is known to interfere with multiple email clients (not just Thunderbird).
Some affected users have been able to resolve this issue by accessing the Avast settings and disabling Mail Shield so that it no longer interferes with an active mail client.
If this scenario applies and you are unsure how to disable Avast’s Mail Shield, follow the instructions below:
Open Avast on your computer. You can do this by double-clicking the desktop icon or clicking its icon on the taskbar.
From the main Avast toolbar, click Settings in the left navigation bar to open the Avast Settings window.
Then, in the new list of options, click the Protection tab to view all active protection components.
Accessing the security tab in Avast Settings
Go to the right tab, select Core Shields and select Mail Shield under Configure Shield Settings.
Once you can locate the Mail Shield component, uncheck the box associated with Enable Mail Shield and then click “Stop forever” in the new menu to turn it off. Thunderbird connection to server was reset
Disable Mail Shield
Note. If you want to temporarily disable Mail Shield to make sure it’s caused by your security suite, you can select Stop for 10 minutes, Stop for 1 hour, or Stop until your computer restarts.
Click OK to close the Avast Preferences window.
After disabling Mail Shield, repeat the action in your email client that previously caused the issue and see if the error continues to occur. Thunderbird connection to server was reset