L'envoi du message a échoué Le message n'a pas pu être envoyé car la connexion au serveur sortant SMTP relay. Proximus.be a expiré.Veuillez essayer à nouveau.

Post a screenshot of your outgoing (SMTP) server settings, so we may verify they're correct.

Read the following article in the meantime to see possible causes of the problem:

https://support.mozilla.org/kb/cannot-send-messages

In particular, if you're using a VPN, disable it and see whether that makes the problem go away. If it does, check whether your VPN has an option called split tunneling which would allow you to let Thunderbird bypass the VPN.