E-mail addresses without user names

Every so often I get spam from an address like @hotmail.com or @yahoo.com with no user name at all. I know why these kinds of addresses are used - they can’t be blocked - but:

A) How do spamsters get an address without a user name?
2) How do spamsters log in to check their mail?
C) Why go to this amount of trouble when the user nameless address will soon be abandoned like any other spam mail address?

A) They don’t. They’ve spoofed the header to make it appear that way.
2)[sup]smartass[/sup] There’s a real return address (that they often don’t check, anyway), but it was erased by the ‘spoofing’ process (Don’t ask me how this is done. I’ve made a point of not learning so that I am never tempted).
C) Because it takes just a little longer before it’s closed this way. That lets the spammer nail a couple million more addresses.