Anything seems to be possible in the world of spammers.
Most of the spam I get has my address in the "from" box too, and comes from China. I think the idea is that when you Spamcop them (I do, usually) they know who did so and therefore know that address is "live".
I have never given out either of my "real" ISP e-mail addresses, and they've never had any spam. I use several domains, for different purposes (Church, personal, flying, alumni association, and so on) and send with them all as the "from" through the same SMTP server. That SMTP server has secure logon, so I can use it from anywhere - not just when connected to that ISP.
Until recently, I used two Spamcop addresses - one for "direct" stuff to my own domains, that i would report, and a second (no reporting) for stuff that comes via routing servers where reporting would hit the "genuine" domain rather than the spammer. I've dropped the second (saving a whole US$30 a year) and now use Gmail as the filter. That works extremely well - no false positives in the 3 months I've been doing it, and only a couple of spams have got through it.
There was a time when spammers would pick an address at random as the "from" in their junk - and the poor unsuspecting owner would get thousands of "bounce" messages as a result. I don't know if that still happens, but I've not been drowned in bounces for a year or two.