[EXIM] sender_host_reject_recipients again

I've got the following in configure.

sender_host_reject_recipients = "+allow_unknown:dbm;/opt/exim/etc/reject.hosts"

And the reject.hosts database is built with the following lines.

*.ms.uu.net
*.da.uu.net
*.dial.ans.net
*cust*.uu.net
*.pub-ip.psi.net
*-ts-*.dialsprint.net
*pool*.grid.net
*dialup*.mci.net
*.dial-access.att.net

Yet this arrived today.

>From dump142@??? Wed Mar 11 08:44:26 1998

Received: from mail1.toronto.istar.net [209.89.75.17]
        by wu.bogon.com with smtp (Exim 1.890 #4)    
        id 0yCocD-0003Xk-00; Wed, 11 Mar 1998 08:44:26 -0800
Received: from vip by mail1.toronto.istar.net with local (Exim 1.80 #5)
        id 0yCoeS-0004Gl-00; Wed, 11 Mar 1998 11:46:44 -0500           
Received: from 224.chicago-11.il.dial-access.att.net (12.66.115.224)
[12.66.115.224]
        by mail1.toronto.istar.net with smtp (Exim 1.80 #5)                         
        id 0yCock-0003gP-00; Wed, 11 Mar 1998 11:44:59 -0500
From:     dump142@???                                   

So my question is, why? How did it get through. Are the leading
wildcards not needed in a dbm lookup? Should I be using partial-dbm?
I should know this by now but I still find myself a bit confused on how
exactly this works.

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