How's this for ironic.  MailFrontier, who has recently released a beta version of their spam-filtering software, is taking flack for modifying peoples signatures in Outlook to promote their software.

Recent targets of the practice include Web surfers who installed a test version of an anti-spam product from MailFrontier, a Palo Alto, Calif.-based software developer. When Web surfers install its Matador product, the download automatically alters their signature line in Microsoft Outlook to read: "This mailbox protected from junk email by Matador from MailFrontier Inc."