Spam sent to me, reported other "To:" address. How?!?

This one's just out of interest really. Today I received a spam email (which was fairly convincing at a glance actually). It dropped into my mailbox but the To: address was not even the same domain as mine. My question is how the hell have I received this, it should have been rejected at the server right? (The server in question is Exchange 2007). I've tried recreating the email through telnet and do get an "unable to relay" message which is what I'd expect. Here are some of the email headers:

Received: from [120.28.125.171] by web163602.mail.gq1.yahoo.com via HTTP; Tue,
 01 Oct 2013 21:48:50 PDT
From: "auto-confirm1118@amazon.co.uk" <woolseyvioleta@yahoo.com>
[b]To: Piotrusd <piotrusd1@wp.pl>[/b]
CC: Mustafa Kaiser <mustafa.kaiser@me.com>
Date: Wed, 2 Oct 2013 05:48:50 +0100
Subject: Your Amazon.co.uk order #-:7-2193465-4169335

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You can see the random 'to:' address, my address is not mentioned once in the source of the email. My reason for wanting to get to the bottom of this is this type of thing should be blocked unconditionally in the future, it's only a matter of time before a company credit card or something is given over to these people...

I hope someone has some insight into this type of thing.