Activists who's email addresses bounce or who report your email as spam to their email service provider will be automatically unsubscribed from your email list. You can tell an activist is bouncing or has complained about spam by looking at their record in the Search & Add Activists section (a yellow X means bouncing or spam complaint, hover over for a few moments for more info), or by pulling a report for those subscription statuses, or by looking at the subscription status field, again in reports.
Activists who bounce or mark a message as spam are also put on a global block list our email deliverability provider maintains. That means even if you resubscribe these activists, they will simple re-bounce when you send another email to them when they hit this global block list.
To resubscribe these activists, first you must contact us at email@example.com and request they be removed from the global block list.
We are happy to remove bouncing email addresses straight away. If an activist has reported a message as spam, however, we require that the activist email us directly with a message indicating they understand what they did and pledging to never mark email as spam again. Only then can we remove them from the block list.
Once they have been removed, you can resubscribe them by doing an upload, checking the resubscribe checkbox, as explained here, or by clicking the "Subscribe" button in their activist record.