No To Header

Hello all,

I am new to procmail and I am looking for some information.  We get tons
of spam through one provider of the following form:

        Date: Mon, 4 May 1998 00:30:49
        From: more(_at_)success-network(_dot_)net
        To: unlisted-recipients:; (no To-header on input)
        Subject: Maybe you can help us


Can anyone tell me how to filter mail that doesn't have the To header?  At
what point is the error message generated?  Will a simple 

        * !^To

line work?

Thanks for all the help.

S. William Schulz