If you've received a mail from SpamAssassin, and you want to know how and why, please visit this page.

The key point is that the most probable situation is that your ISP has set this up on your mail account, and to switch it off, you MUST contact their Tech Support desk. Nobody in the SpamAssassin development team can do this, ONLY your ISP can.

Otherwise, please visit the new SpamAssassin FAQ Wiki here.