English text for the error message: An error occurred while sending mail: The mail server sent an incorrect greeting: 5.7.1 <unknown[2a01:cb10:1ae:1400:c587:d95b:ce2d:4a1c]:43100>: Client host rejected: Access denied. I think the most obvious is that you may have selected a different SMTP server for your account, or a different [[Using Identities|identity]] that has another SMTP server configured, so the server simply refuses your account credentials. Make sure to choose the proper SMTP server at the bottom of your account setting on the right, when the account name is selected on the left. Also make sure to use valid SMTP server settings and credentials for configured identities. Looking at the IP, it may also be that your client is trying to connect using IPv6, but the system and/or ISP only allows IPv4. Did you or your ISP change anything related to IPv6? Have you started using some proxy add-on perhaps? Another option is your ISP moved to a different way of authenticating, which could involve OAuth2 or other authentication settings. For more on that, please you have a look at [/questions/1166437 this question] for a possibly similar issue and solution. Another option would be that your system or IP has been flagged for sending spam or abuse. Make sure your system in not infected by malware. See [https://www.massmailsoftware.com/smtp/554-5-7-1.php this page] for more info - you can enter your IP (v4) number in one of the provided blacklist checkers and contact your ISP if needed. In rare cases nowadays, you could also be connected to another (open) WiFi network which will prevent you from accessing your ISP’s SMTP server if no authentication is configured in Thunderbird. That would generally result in another error message, but some servers aren’t very consistent in handling them. Please let us know if anything helped.

Thank you very much ! Have a nice week-end Jean-Luc