The transactional email wasn't adding someone to a newsletter. But you can mark 'spam' on any email at all, and some people felt that our anniversary-of-them-signing-up emails or "you've had a problem being billed" emails were spam, so they hit the button. idk man! People are crazy.

We also had marketing emails through mailchimp, but they were totally separate systems.

Ah. I'd have marked the former as spam too (it's an advertisement reminding the recipient you exist, not something in response to a transaction just committed / something they did), but the latter seems plenty safe.

But then, I have no idea how you'd be correlate the transactional mail to the marketing mails anyway (I'd use separate addresses for that). If somebody added my address to a list I didn't sign up for, any mail from them would just be a reminder they did so... It wouldn't be hard to imagine somebody marking everything in the future as spam automatically after glancing at the sender name.