Receiving emails not addressed to you
How come I received this message? My address isn't in the To: header.
The mail system does not use the contents of any email header to route mail. (Actually, some bits of email software do, and they are broken and cause terrible problems. Thankfully, most email software is not that broken.)
A mail message in transit should not be routed with reference to any email headers. The mail systems pass the destination addresses between themselves as part of the mail transfer protocol, but this information does not appear in the mail headers themselves. This allows the destination addresses to change - for example, because of forwarding operations or by going via a mailing list - while the addresses in the headers remain the same. When a legitimate, well-behaved message is sent, the addresses in the message headers represent the addresses to which it was initially sent, but the final destinations may be quite different.
The classic example of this is a message sent to a mailing list. If you're on any mailing lists, look at the message headers closely. The To: (or maybe Cc:) header has the address of the mailing list in it, but not your individual address. Yet the message is delivered to you. So the To: header does not have to match the recipient address.
Spam is not well-behaved. Often the addresses in the To: and Cc: headers have no relation to your addresses at all. If this is difficult to understand, think of it as though you had had the message Bcced to you. The effect is more or less the same.