You mention the word 'SPAM'. If that indeed is the word being shown then this is not been done by Thunderbird. Either you have another program marking emails as 'spam' eg: SpamAssassin or this has been done by the server. If the headers say this is spam, then it is likely the server spam filters could be blocking it. Be wary of sending spam labelled emails as your email address could end up being blocked because you are sending spam. Suggestion: you create a new email and attach the spam email. So sending it as an attachment. However, where is the 'Spam' email ? Is it in the 'Inbox' or in the 'Junk' or in the 'Spam' folder? Is this a pop or imap mail account? If the words used a 'Junk' (not 'spam') then you could select email and mark as 'not junk' to return it to the Inbox. There is a difference between Thunderbird filter thinking something is 'Junk' and the server marking it as 'Spam'.