Error message : ''L'envoi du message a echoue. Une erreur est survenue lors de l'envoi de l'email. Le serveur a repondu: Email ........rejected per Spam policy. Veuillez verifier le message st essayer a nouveau'' In English - please help us by translating error messages as it takes us a lot of extra time trying to help loads of various people. ''The message sending failed. An error occurred while sending the email. The server responded: Email ........rejected per Spam policy. Please check the message and try again.'' Basically, the server says that 'Reply' looks like spam so it's rejecting it. This is not Thunderbird failing to send - it did send to the server, but server says if email looks like potential spam then we will not allow it through our server. Spam - There's a lot of info out there on how to make email not look like spam. Usually spam emails contain several links whether they are text or images. Probably because it contains the complete email signature of the original sender. You do not need to send someone elses signature back to them especially if contains remote content or links etc. The text on the far left should be ok where it says P/o Maitre Nicolas Peyrat But there are images and links in addition to that text. Edit the Reply message to remove the original senders signature section such as the area whee it says Notaire in the middle and the image and text ..Par le temps Then attempt to send.