Well, Thunderbird is not your email provider; it's just the agent. If the settings are incorrect, the ideal group to contact would be your email provider, although many of those give incorrect or incomplete information. If you post a screenshot of your SMTP server settings pane, and translate the error message completely to English, someone here may have suggestions to assist you.

Okay, my provider is a private server, so I'll try and contact the person in charge. I'm sharing my settings panel and the translated message is below, just in case. Thank you very much. "Sending the message failed. The outgoing server (SMTP) mx.thdo.eu does not seem to support encrypted passwords. If you have just set up the account, please change the “Authentication method” to “Normal password” in the “Account settings | Outgoing server (SMTP)” menu. If this worked before and now fails, please contact your e-mail administrator or provider."

The email provider should be able to answer this. Sometimes, depending on the email provider, I have found that experimenting with other settings, such as port 587 and STARTTLS, works. I wish I could be of more support.

Hello, I got it all right. The person in charge of the server made some changes indeed, and guided me through the modifications in the settings. Thank your for taking the time to answer me !

I am pleased that it worked out for you. :) Thanks for sharing.