[exim] extract ip from Received lines in ACL

Hi

On my system most spam/scams that escapes filter comes from freemail
addresses, and i would like to filter harder on mail that comes from
countrys like Nigeria, China etc. Hotmail and others have
X-Originating-Ip: header that i can use, but yahoo puts ip in received:
header, is it possible to grep the ip from the last received line within
a acl?, like this
Received: from [81.91.232.194] by web37004.mail.mud.yahoo.com via HTTP;
Sat, 12 Sep 2009 21:15:38 PDT