You get a message from your provider the message is detected as spam. Yes? I make that statement in the knowledge that Thunderbird does not do anything with outgoing mail with regards to spam. What I am seeing is a significant uptick on poorly trained folk with ISP's implementing spam filters that simply block all of their customers submitting mail to their SMTP server because the IP address the ISP has allocated is a domestic one, not a fixed business one. These block lists exist to stop spam bots in malware sending spam. But the providers that are implementing them apparently forget their customers are also legitimately submitting mail. Usually they need to limit the block list to port 25 submissions, but apparently miss that part of the discussion and apply it universally with poor results.