Create a filter that disposes of (i.e. deletes) messages where your address doesn't appear in the To: or Cc: box. But be prepared to lose messages where you have been Bcc'd. https://support.mozilla.org/en-US/kb/organize-your-messages-using-filters The messages are probably being sent with your address in the Bcc: field, which you as a recipient won't get to see. What you do see are the addresses placed in the To: and Cc: boxes by the sender.