Recently, Apple added a feature in iOS 10 that allow you to unsubscribe from a mailing list from a header in your mobile email application.
As far as the Sailthru side, when a user Unsubscribes through this header, it is sent through a feedback loop to Sailthru that initiates an "optout-basic" on that users profile. This is also the same process for the similar Gmail Unsubscribe header that was released a 2 years ago.
At the present, Sailthru does not have a way to capture these users in Audience Builder, you will need to manually aggregate this by downloading the desired user list, and searching for users that have unsubscribed with the "Reason: unsubscribe-email" tag. This will also include all Apple domains as well as any other provider that offers this feature (gmail, yahoo, apple, etc).
Locating These Users
At this time, Audience Builder is unable to decipher between users who optout using this method versus those who optout using other methods. To compile a list of users who opted out using the List-Unsubscribe option in their email account, first export all users in your Sailthru account making sure to select “optout data” when you export
Once the export is complete, open the resulting csv in a spreadsheet application such as Microsoft Excel. Sort the csv looking for the "Reason: unsubscribe-email" value. This signifies that they used this List-Unsubscribe link to optout. This would include all email providers that offer this optout option.
If you have any further issues, please contact Support@sailthru.com.