Just because someone has unsubscribed from your mailing lists doesn't necessarily mean you want to stop sending them all mail. For example, we've got people who unsubscribed from our mailing lists but still submit support requests by email, using the same email that had been on our mailing lists, and they expect to continue to get answers to those requests.

This seems like a solvable problem? I would think that it wouldn't be hard to mark support emails in a way that didn't overlap with marketing emails.