Mickey Chandler asks, "Do you want to fund the lawsuit?" Meaning, if you're splitting the hair very thin to make the case that what you're doing *must* be legally compliant, ask yourself: Do you really want to be that edge case that gets pulled into court? If not, do the right thing and back away from the edge. It's not about what you can get away with under the law. It's about following best practices, not sending spam, making it easy for people to unsubscribe, and doing what ISPs want.
If you're asking yourself, how close can we get to the line without stepping over, it may well be that you're asking the wrong question.