Spam warning in subject
From MozillaZine Knowledge Base
If somebody complains that a message that you sent them has something like "[POSSIBLE SPAM]" in the subject, this is not due to Thunderbird. Thunderbird never uses the word "spam", it uses the word "junk". "[POSSIBLE SPAM]" etc. in the subject is probably due to your recipient's email provider running some anti-spam software. A lot of corporations do this. Some less likely possibilities:
Some users report that this problem occurs when they use Thunderbird but not Outlook Express. Anti-spam software can be very quirky. For example, some anti-spam software treats any message that (correctly) identifies the email client as "The Bat!" as spam, even though "The Bat!" is a well known email client. . Assuming both Thunderbird and Outlook Express are configured to use the same SMTP server one possibility might be that the anti-spam software has problems identifying the email client. This typically occurs if there is no X-Mailer header but some SpamAssassin rule sets for example will increase the score if both X-Mailer and User-Agent are set while some others will increase it if only X-Mailer is set.  These headers are normally not set by the user, they're due to what email client you're using. And there is always the possibility of it not recognizing the X-Mailer header. Usually this shouldn't be enough to cause your message to be classified as spam but your spam score might have been close to the limit due to using a SMTP server from a email provider that has a reputation of sending lots of spam plus having something unusual about the message body. Typically you don't know the actual spam score so you have no way of telling how close to the limit the message sent from Outlook Express was, just that it was not labeled [POSSIBLE SPAM].
Ask the recipient to forward you the message as an attachment and see if it added some custom headers to the original message that explain why it thought it might be spam. For example, if the email provider uses SpamAssassin it might add the following headers:
You could look up what the strings in X-Spam-hits mean at http://spamassassin.apache.org/tests_3_2_x.html .
Your spam score (how likely a message is spam) is based mainly on the content of the message, but what SMTP server you use counts. Try not to use a SMTP server from Yahoo, Hotmail , MSN or a large ISP such as Comcast or Earthlink if you have multiple email providers.