How can email possibly be routed to the right place with no to: address?

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http://serverfault.com – I'm no novice on networking technology, but one thing I don't really know much about in detail is email and headers. How does email work SPECIFICALLY? I'm getting spam in my hotmail inbox when I've made painful attempts to not give out my actual email. I use my own domain name to forward email to my inbox using several aliases. Yet now I'm getting spam with no address in the to: line, or also "undisclosed recipients". Looking at the headers is of no help whatsoever. So from a technical standpoint, I have to wonder... if I send an email to a certain address in my personal domain and it gets f (HowTos)