Intuit Quickbooks is another service that gets used like this. My mom received some fraudulent invoice from an Intuit Quickbooks user, which was from the legit Intuit domain since it was sent by the service, which claimed that she owed "PayPal, Inc" $600 for something or other. I told her to delete the email and reported the activity to Intuit but never heard back.
I literally received one of these today. When I clicked the spam button in Gmail it asked not just if I wanted to "mark as spam" but also "mark as spam and unsubscribe." I clicked the latter, and it took me to a seemingly real intuit.com url. It was pretty weird and I wasn't sure how the spammers pulled it off. Seeing this article is real serendipity.
