As the message says, it is coming from a mail server. So it's not from Thunderbird and it's not from your own computer: most likely, it's from the server of the mail provider used by those recipients. Are they all on the same email domain? Anyway, for some reason your message is being classed as spam. That might be because of something in your message or because the company that provides your email service is used by others to send spam. The best way to resolve the issue is probably to contact the recipients another way and ask them to raise it with their email provider. They may be able to 'white list' your email address so that messages from you are not subject to the usual spam filter.

Thank you for you quick answer. So It is a problem of the sender's provider if I understand it right?

I may have misunderstood you. I thought you were sending the email. You said 'in the reply action', I assumed you meant that you were sending a message in reply to one that you had received. My explanation is that if you are replying to a message that you have received from ' ... @some_domain', and if you get a server error saying that your reply is classed as spam, then the error message is probably coming from the 'some_domain' server. And the best way to resolve it is to contact the people who use that server. I hope that clarifies!

It does.....thank you very much!