X-Spam-Relays-External

[Ned Slider]

Hi List,

I see the useful X-Spam-Relays-External pseudo header but what I'd 
really like to be able to specifically check is the Last External header 
as DNSBL rules are able to do with -lastexternal.

Is there a X-Spam-Relays-Last-External option that I'm missing, and if 
not would it be possible to implement such a feature or perhaps someone 
can suggest a workaround method?

For example, I'd like to be able to do something like this against only 
the last external Received header:

header __RCVD_FROM_SOMEISP  X-Spam-Relays-Last-External =~ /someisp\.com/i

Thanks