Whenever you send an email campaign, it helps to include a quick permission reminder for people who might have forgotten that they subscribed to your list. Otherwise, some of them will be quick to report you for spamming.
Some email firewall companies specifically look for this "Permission Reminder" text in your campaign should one of your recipients report you for spamming.
MailChimp inserts your Permission Reminder into all our built-in templates for you automatically with the *|LIST:DESCRIPTION|* tag.
You can update your Permission Reminder text by following these steps in your account: Lists Tab ==> settings ==> Abuse Info & Permission Reminder
You will need to follow these steps in our system and update your Permission Reminder with something like: "You are receiving this email because...." and then tell specifically how they signed up and came to be on your list.