E-mail, chat, & VoIP : I have a "different" email question....

I have a "different" email question....

I have a different kind of email question. First, a little background: I use MailWasher Pro, which I find fairly accurate at identifying spam, so I'm happy with it. All spam that I receive is automatically marked as such, and I NEVER download it into my email program, Thunderbird. However, on occasion, I do preview a spam email in MailWasher, out of morbid curiosity I guess, and I always take a look at the header information to see if there is any hint as to how I may have gotten on a particular mailing list. Lately, I've been getting a lot of spam which, according to the header info, doesn't seem to be addressed TO anybody! There is simply no "TO" line at all, or if there is, it's almost always "TO" someone other than ME. I got one today that was addressed "TO" the same person it was allegedly "FROM" ! My question is, HOW DO THEY DO THAT? If the email has NO "TO" address, how does it wind up in MY inbox? Or anybody's, for that matter?! As I stated earlier, this is mostly just my morbid curiosity working overtime, LOL, but if anyone can educate me as to how this is accomplished, I'd be very appreciative. Thank you!