Q: How did they get mail address?

A: Spammers have programs that generate millions of email addresses for any domain. Then they mass mail all the addresses not caring who they go to. The mass mailing program will repeat this as many times as they set it to. This is why you may receive the same spam 4 or 5 times in a day.

Black listing these emails will not help because the address they are using to send the spam is not real.