Please provide more information - exactly which server does the error mention? What process causes the error - downloading or sending? What email provider, what antivirus (AVG?), what authentication type? Was anything changed in your email account online or in Thunderbird? Meanwhile try these IF they fit your situation. Note that solutions differ for different email providers, and whether you use VPN, and your antivirus settings may also be affecting it, so none of these may work for you, they are just a starting point: https://lucianwebservice.com/how-to-tutorials/thunderbird-connection-to-server-pop-gmail-com-was-reset-error-solution.html https://forums.att.com/conversations/att-internet-email-security/email-connection-reset/5df0227bbad5f2f60664c99c