Ok so your mail provider has examined you email and decided it is spam and refused to send it. (that is if you get this message after clicking send) Have you asked them what the error message means and how to prevent it in future?

If it comes back later as an email you will need to include the content of the return message including the server sending the message.

If you message is an image, try adding text before the image in the email and after it.

I don't know the origin of the message - Thunderbird? or my ISP?

Thunderbird never attempts to validate an email. It serves only as your messenger.

The origin of the message is not Thunderbird.

If it appear in a dialog right after you click send, it will be from your mail provider. If the message comes back as a returned email, it will probably be from the receiving mail server.