[exim] Hotmail spam issue

Hi,

We get some spam over the Hotmail.com network. The only way that I can
tell where the mail is actually originating from is the originating ip
header that hotmail adds to the email which looks like this

X-Originating-IP: [41.219.194.12]

What's the best way to have exim set this ip address into a variable
such as acl_mx? I've looked at match and sg but I can't put this
together.

Thanks.
Craig Jackson