If a subscriber has an Inactive status, messages are NOT sent to that email address. This might be because they have had too many bounces (3 consecutive bounces) and Mustang List has rendered them inactive because of this (you would see the number of bounces in until Total Bounces to verify if this is the case). Or the subscriber may have unsubscribed, in which case the record is turned to inactive.
A record that is inactive can only be rendered active again by manually changing the status. A subscriber who had formerly unsubscribed can sign up again on your online form. But you cannot re-import a subscriber who is in the system with an Inactive status. This is to prevent subscribers from being “re-subscribed” against their will. You can use this feature to your advantage: if there is someone that you do not ever want to add to your email list, you might add them to your list, but turn the their status to Inactive to prevent them ever from accidentally being added during an import.