I've been accused of spamming but these people legitimately signed up for my newsletter. How did this happen?
If you have had an AUP violation raised against your site, you will still need to take immediate action to keep your account online, but hopefully the following will help you understand what happened, and how you can avoid a repeat of the situation. (Or help keep it from happening in the first place)
Is a third party advertising through you? If so, your client probably got your affiliate's email and didn't recognize it as coming from you. Since it looked like spam, it was likely reported as such.
Make it very clear that each email is from YOUR site instead of another. Clients will then remember that they signed up for the service and will be less likely to report you as a spammer.