I used to work at a "very large email advertising company". We would take another companies email list and advertise to it, splitting the profit with the compan

I used to work at a "very large email advertising company". We would take another companies email list and advertise to it, splitting the profit with the company that owned the data. When an unsubscribe came in- sure, we could remove it from our own list, but we'd still have to give the unsubs to the original data owner, who'd need to then send the unsubs to any other companies that were using their data. And that's at the simplest, it can run multiple levels deep (i.e, Company A sells data to Company B who sells data to Company C). Another reason is that "targeting" emails on a user-by-user basis can take lots of processing time, too much to be done in real-time when the email is being sent out, so they are often queued up several days in advance.