Outlook Express - why do I get spam sent to another address?

Seems to me that the whole email business is so complex and arcane that very few folks, including my ISP, fully understand it. My provider's helpdesk people (situated somewhere in the jungles of South-East Asia, apparently) certainly don't understand the system, and even Mr.Gates seems powerless to do anything about spam.

Case in point: Today I received an Outlook Express email. The preview pane header reads:
From: Marcelo Rojas To: colin.walter@xnet.co.nz

Now, I have no clue as to who Marcelo Rojas might be, which is to be expected. But the apparent recipient, colin.walter, is NOT me.

OK, if I right click the email listing in the upper pane and select Properties > Details, I can see Marcelo's address, officials@remaaz.com, which may or may not be genuine. And there, following the words "Delivered to' is my real address.

So where does "colin.walter" fit into the equation? Why do spammers insert these 'third party' labels when it's such an obvious give-away that this is junk mail? I mean, I'm far more likely to read something addressed to me than to someone unknown.

Another annoyance that really bugs some of my customers is when they receive emails that are purportedly from themselves. This isn't too hard to fix with a Mail Rule that consigns all mail from oneself to a separate folder, but it's a nuisance, nonetheless.

As I said, I have a feeling that the whole system is so complicated that it's only fully understood by the very few people who've found a way to use it to their advantage.