If you are managing your lists outside of BoltMail, such as in a spreadsheet or other third party system, then you will need to remove unsubscribers from your external list.
You can export your unsubscribers from Lists > All Subscribers > Filters, then in the filters form set "Status" to "Unsubscribed" and set the "Action" to "Export". Then click Submit to start the export.
The filters screen contains many other options for filtering specific subscribers, such as subscribers who have opened or not opened a specific campaign.
