AFAIK, it works like this:
QUOTE
Received: from a-server.somewhere.com ([55.66.77.88]) by nowhere.com with Microsoft SMTPSVC(6.0.3790.1830); Wed, 11 Oct 2006 12:45:28 -0700
Two things can happen:
A ) If spamcop knows that 55.66.77.88 belongs to XYZ ISP and that it correctly stamps it received: from headers, then it trusts this header line and goes on to the next.
or
B ) On the other hand, if 55.66.77.88 is a known open relay that will send anything it is given, then anything beyond this line is POTENTIALLY forged and cannot be trusted, therefore 55.66.77.88 is considered the source of the message and if enough instances are seen, is added to the SCBL.
If option A occurs, then spamcop goes on to the next received line:
QUOTE
Received: from b-server.somewhere.com (b-server-34ds.somewhere.com [33.44.55.66]) by a-server.somewhere.com (Sun ONE Messaging Server 6.0 HotFix 1.01 (built Apr 15 2004)) with ESMTP id <2A35FAS2LMJJHG0[at]a-server.somewhere.com> for me[at]nowhere.com; Wed, 11 Oct 2006 13:47:07 -0600 (MDT)
and repeats the same process as above. If 33.44.55.66 can be trusted, it moves on, otherwise it considers 33.44.55.66 to be the source.
QUOTE
Received: from bill ([11.22.33.44]) by b-server.somewhere.com (Sun ONE Messaging Server 6.0 HotFix 1.01 (built Apr 15 2004)) with ESMTP id <2A35FA32LKJLL99[at]b-server.somewhere.com > for me[at]nowhere.com; Wed, 11 Oct 2006 13:47:07 -0600 (MDT)
If all prior received lines are trusted, then the final line is processes and 11.22.33.44 is considered the source. Again, this does not mean it is automatically added to the SCBL after one message. A number of other factors are taken into account, like:
How many messages is this server sending?
How many reports are there against it?
How many spamtrap hits are there against it?
Does it have a prior listing history?
Those are the ones that we know of, and I'm sure there are other criteria that are taken into account before the SCBL listing shows up.
Now, if you are actually asking about the FILTERING process at a receiving ISP, then that is a bit different.
In your above example, most ISPs will not look at the headers at all when considering filtering, as that means they have to commit to receiving the message and paying for the bandwidth. Most ISPs will simply look at the IP address of the server that is trying to send the message, and will make a delivery decision based on that IP address.
I believe the spamcop email system is an exception, it examines all header lines in a message for "bad" IPs and considers those in its weighing of the message as potential spam. There are other mail systems that work like this as well, however, I believe they are the exception rather than the rule.