First off Thunderbird has nothing to do with "my server" what ever that means. It is software that runs on your computer. Secondly it does not use the word Spam or modify the subject line to include the word Spam. Thunderbird uses the word Junk to differentiate itself from your email providers spam filter and your antivirus spam filtering that does mark messages as spam. You need to look at your providers spam filter or antivirus software for the source of this tagging.