Hi Alice,
Campaign can only track one types of inbox movement which is if someone reports the email as Spam.
This is reported back to Campaign using Feedback Loops which has to be setup for your instance by deliverability consultants if not already done.
Now not all email providers have a feedback loop but the major players do have it. Gmail, Hotmail all have this feedback mechanism.
Once you receive the FBL information, the user is marked as blacklisted by Campaign.
Regards,
Vipul