XP Pro SP 2 / OE6 / IE7

Getting a lot of spam where the targets are evidentally on the Bcc:
header, because the To: and Cc: headers are empty.

Like to send that **** to the Trash. I thought it might be as simple
as creating a rule ' where To: contains "" ' but that doesn't seem to
be it.

Any suggestions, or known ways to specify this?