why did this match?

I've had a mis-hit on my spam-filter, and I can't figure out what matched.

Here's what I saw:

X-SpamChecker: Blocked Domain: from phord (user-37kb6o3.dialup.mindspring.com

Here's the relevant header (formatted for your convenience):

Received: from phord (user-37kb6o3.dialup.mindspring.com
        [207.69.155.3]) by emmo.speedsoft.com (8.8.5) id TAA18949;
        Sun, 19 Oct 1997 19:27:15 -0600 (MDT)


Problem is there is NOTHING in my spam filter that matches anything AFAI can 
see.


% egrep "emmo|speedsoft|phord|user-37kb6o3|dialup|mindspring"  
spam-rc-files/domains.rc
exit 1 egrep
%

domains.rc looks like this:


:0fhw
* ^(From.|Reply-To:|From:|Message-ID:|Received:)()\/.*(1-global\.com|\
        123go\.net|\
        1daystar\.com|\
        1stchoice\.com|\
        1stfamily\.com|\
...<snipped for brevity, it's 1339 lines>
        zontroop\.com|\
        zrs\.net|\
        zwag\.com)
| $FORMAIL -A"X-SpamChecker: Blocked Domain $MATCH"


How can I investigate this further?

Thanks

TjL