After we changed a provider of our e-mail box we have a problem with sending e-mails. Thunderbird frequently displays the error "Sending the message has failed because the connection to the server has timed out", The error is displayed more often when replying to messages, and less often when sending a new message. The error is displayed regardless of whether the message has an attachment or not. We tried to change the wait time from the default value of 100 to 140. We also tried to change the minimum TLS version. Configuration is carried out as recommended by the provider. Other e-mail programs are sending correctly. We contact with our provider they say that everything is ok on their side. Thunderbird is updated to latest version, windows version from 2004 to 20h2, we are using the ESET endpoint security version 8.0.2028.0. We generated new eset certificates for thundebird but that did not solve the problem, also we tried to disable/uninstall our antivirus and add thundebird to the exceptions in both the antivirus and our firewall. Uninstalling thunderbird, deleting all folders, and then reinstalling and configuring does not work. Antivirus : ESET Endpoint Security version 8.0.2028.0 Mail provider: idhosting.pl smtp username is the full name of the email address for example [email protected]