Say I was a spammer and had ten thousand email addresses. I wouldn't want to share them with anyone and also if any of my addresses were wrong, I don't want to be notified.
What I would do is to pick some poor sucker from my list and address the email to them. The other 9,999 addresses go in the BCC field, so no-one else can see them. Then I mail it. Any addresses that bounce back will go to the person in the TO field and not to me.
Bingo! 10,000 spam emails sent out and no-one thinks it was me!
You are in the BCC list of the spammer's email.