<<Si è verificato un errore durante l'invio della posta: il server di posta ha iniziato la conversazione in modo non corretto: connection not accepted from blacklisted IP address.>> i encountered an error while sending mail: the mail server started the conversation incorrectly: connection not accepted from blacklisted IP address. The error message means that either your computer IP address is blacklisted or your ISP server IP address is Blacklisted. This is not a Thunderbird issue, but obviously you will not be able to send. Some helpful links: * https://www.whatismyip.com/why-is-my-ip-blacklisted/ * https://whatismyipaddress.com/blacklist-check Try changing your IP address to see if this helps.